Sure in many cases one has to look at how much something is popular relative to the general population, but do you really need to think like that when talking about websites and forums? I mean, from an absolute point of view there are many more forums, personal blogs, rss feeds and independent projects than ever before, even if their relative popularity has declined. But what do you care? A forum hosting 300 active users is already incredibly crowded, it's very difficult you will get to know even half of all them in your own lifetime, why do you need everything to turn virally popular?
Would you want your local NIX club to suddenly jump from a few tens of members to thousands? No, because the point is to create a community. Who cares if some NIX club residing on Facebook has 100.000 members, ultimately what you care about is the one you go to every week/month and in which you see real people and talk real talk. We are not talking here about an industrial endeavor where if not enough users partake then production crawls to a halt worldwide, we are talking about buying a crappy computer, coding a few pages in simple HTML and buying a funky domain name for pennies. It will always be cheap and simple to host your own stuff, you don't need self hosting to become the next big thing to enjoy your life.