It is said on Stack Exchange Company Page that the company is behind the largest technical community on the Internet. How was that determined?
StackOverflow is #60 currently. You could say it's market-ese, but no higher ranked site is even arguably a "technical community." You're only rival would be sub-communities within larger sites, such as Facebook, but to my knowledge nothing like that really exists at large scale (and if it did, being at large scale, I would probably have heard of it).
In short: It's market-ese in the sense that SE didn't really need to be rigorous to make that claim. But any "reasonable person" attempts to be a little rigorous are consistent with their claim. So, I don't think it's misleading or wrong or disingenuous. Perhaps when some prominent rival (...like what?) makes that claim the two titans will start busting out metrics and rubrics and such.
I agree Alexa is not a good measure of "community size" although it made a good rubric by which to illustrate. It's not at all my intent to go through whatever metrics we can come up with; I strongly suspect SO will pass by a "reasonable person" test any metric that itself would pass a "reasonable person" test.
Regarding the word "technical" please note the full claim:
As the company behind the largest technical community on the Internet, we've been able to attract some of the top developers, sysadmins, and designers in the world.
Which very clearly puts the focus of the word "technical" on technological matters. You could maybe sort of try to argue that IMDB counts as "technical" in the sense that it involves domain expertise - and only in the sense that they are discussing technical matters of cinematography, make-up, etc. and not movie and film knowledge in general - but the claim is clearly not interested in that liberal definition.
How was that determined?
It was determined by the Marketing department at Stack Exchange. You won't find a list of "largest technical communities" anywhere.
If there was one, Stack Exchange would certainly be near the top if not the top site with it's impressive near 2 million registered user accounts (note that many users have multiple accounts).
The thing is, on the company page, the words were chosen wisely.
Having special skill or practical knowledge especially in a specific field.
An interacting population of various kinds of individuals (as species) in a common location
Both these terms are open to various interpretations and that's why they were used. Each Q/A site represents a community of regular registered users posting questions and answers related to a specific field about which you must have certain knowledge to participate. So it fits the description.