I have encountered in various chatrooms discussions whether recently (for obvious reasons) there has been decrease of the activity on a specific site. (Or that some site has less visitors or less active users as usual.) Of course, the question of measuring increase/decrease of activity might be interesting at any time, not only when there are some external factors which are likely to influence the site's dynamics like now.
Question: What are reasonable ways to find out whether there was increase/decrease of activity or number of visitors on a specific site? The same question applies to the network as a whole.
I know that various stats are available in the list of all sites - but this only helps if we also know what the numbers used to be in the past, so that we have something to compare with.
Users with sufficient reputation can check site analytics to see how some statistics (number of posts and votes, traffic) evolved over time.
Are there also some other statistics that can be useful for this purpose? For example, some SEDE queries which return some numbers that could serve as a reasonable proxy for activity or number of visitors and active users?