Why
Site participation depends on each site's circumstances. You could look for the reasons on the per-site / child Meta. Some cases could have "escalated" to this site, Meta Stack Exchange, and some have even reached the news / external sites.
Some events that had an overall relevance on Stack Exchange are mentioned in the Wikipedia article. As of November 21, 2023 this article has two notices. One general notice suggested splitting the article. Another on the section "Declining relationship between users and company" with the following text (links are not included):
This section may contain improper references to user-generated content. Please help improve it by removing references to unreliable sources, where they are used inappropriately. (March 2022) (template removal help)
What
Stack Overflow's Help Center will be used for links because Stack Overflow is the Stack Exchange Network flagship. To find the corresponding help article of another site, replace https://stackoverflow.com from the link URL with the site's address—the same for other pages like Users.
The elemental user activity, referred to as participation, on a Stack Exchange site is done by posting questions and answers. Other actions like edits will "bump" posts to the Homepage, but there are some rules.
IMO, the most critical participation is moderation, not by people with "super powers" but by the site's users based on the Reputation System.
The activity that builds a sense of community goes far away the Homepage
Many things will not be shown on a site's Homepage. Many of them are more important than questions and answers, which foster a sense of community, engage, and make people commit and have a feeling of ownership, to make things happen. Those things motivate them to moderate their site.
Moderation might be referred to in many forms and seen in many places. Not all the moderation outcomes will cause an output visible on the site's Homepage as question, answer or edit.
Besides activity on the main site, look at the site's Tour, Help Center, the per-site / child Meta, the site chat, aka, "The Third Place™," and the off-site places where the users do other things than posting questions and answers and editing posts according to the Stack Overflow Q&A model and rules.
Where
As of November 22, 2023, the Stack Exchange button on the toolbar of this page says that there are "183 Q&A communities".
Stack Exchange Network Stack Exchange network consists of 183 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers.
In the above text, IMO, the term "Q&A communities" refers to "online community" as a synonym of "virtual community". It includes a claim, which is done from a marketing/promotion of a product writing style, so take the communication intention, promoting a product; it's a free digital service, but it still is promoting a product from a for-profit business.
It's perfectly fine to have as an aspirational goal and use it in publicity and public relations: the intention to achieve the sense community, as the Malcolm Baldrige Excellence Award has defined in their category for Communities, as well other institutions have done as something causing satisfaction because the top outcome is the human well-being.
Site and Community
The site creation process has evolved dramatically over the years. Nowadays, to propose a new site, it's required that a community already exists before creating the new site proposal. Also, due to the limitations of what could be done on Stack Exchange and digital services, it's not strange that people look to create meaningful relationships, grow as human beings, and spare their time in other places.
From https://area51.stackexchange.com/faq (emphasis mine):
How do I start a new site?
If your area of expertise doesn't already have a Stack Exchange site, propose it! Stack Exchange sites are free to create and free to use. All we ask is that you have an enthusiastic, committed group of expert users who check in regularly, asking and answering questions.
The creation of Stack Exchange sites is a community-driven process. That does not mean the majority of Area 51 users have to love your site idea. It simply means you have to recruit a community of users large enough so that questions get good answers quickly. Reach out to other experts to build support for your site; bloggers, enthusiasts, and support groups can all benefit from a world-class, canonical collection of expert answers to the hardest questions.
Site
Looking at the output of the Stack Exchange platform, we find sites. The Stack Exchange sites are listed at https://stackexchange.com/sites. This page has two views. The list view shows some site statistics:
- questions
- answers
- answered
- users
- questions/day
- site/age
The above metrics could sort the sites. On top of the list, it's easy to see the most active or the least active. Just choose one of the metrics.
As mentioned at the top of this answer, site participation depends on each site's circumstances.
While a site's Homepage might give some sense of site participation, looking at the Users page might be more helpful. This page has the following views.
- Reputation
- New users
- Voters
- Editors
- Moderators
Several views have filters to show users by week, month, quarter, year and all.
Meta
Per-site / child Metas has its own Users page, with a slight difference. Instead of a Reputation view, they have a Participation view.
Data Explorer
Data Explorer might be used to make a more profound analysis based on data, but according to Glorfindel's answer to How to get the historical data in users Tab
Statistics on voters is not possible via SEDE; votes are anonymized before they are made publicly accessible.
A workaround is to archive Users page views using services like Wayback Machine from The Internet Archive and use archived Users pages to get data from previous / old periods.
Related
Chat
Besides Site and Meta, there is Chat. Stack Exchange has three chat realms,
- https://chat.stackoverflow.com for Stack Overflow
- https://chat.stackexchange.com.com for all other Stack Exchange sites
- https://chat.meta.stackexchange.com for Meta Stack Exchange.
The Reputation System controls access to chat but works differently from others. The privilege threshold uses the aggregation of all per-site user's reputations. Also, chat moderation works differently, so activity and a sense of community might need more study to be understood. Start by reading https://stackoverflow.com/help/privileges/chat.
When
Below is a summary of some of the most relevant events without including those that might be controversial. They are documented in the Wikipedia article linked above.
Stack Overflow was launched in 2008. Later, other sites followed using the same Q&A model/platform. Stack Exchange, later referred to as 1.0, was established.
Server Fault, Super User, the first Meta, and MathOverflow, among others, were launched during this stage.
A new site proposal process was deployed in 2010. For this, the Area 51 site was launched. Stack Exchange 2.0 was established. There was a rule that sites should be "healthy" otherwise, they would be closed.
A bit later, Jeff Attwood, one of the founders, left the company.
At some point later, very significant rules were made. Sites were no longer required to be "healthy" to continue working. Also, it was required for Area 51 new site proposals to be created only when there was already a "community" ready to support them.
Stack Exchange, Inc. was sold. Nowadays, it's owned by Prosus.