This site has an incredibly large scope -- any website that can be classified an 'app' is fair game here. There are millions of websites and thousands more every day. So the scope, if anything, is getting larger and larger over time!
While in practice this means that giants like YouTube, GMail, Facebook, etc will dominate -- and this is as it should be -- it also means that the core user base will be "web app generalists". That is, power web users.
It's hard to form a core community around such a massively broad topic, and it's also possible that people who are heavy, say, Facebook users, may not necessarily be web app generalists, maybe they're just hardcore Facebook users. In which case they'd be better served by, perhaps, facebook.stackexchange.com and youtube.stackexchange.com.
I do think we'd see stronger communities about more specific web apps, but
So, it's tricky. We have an experimental thing coming soon that could help; keep an eye out for it on the blog.