It's a cycle. The internet started with usenet and specific interest newsgroups and niche forums a la phpbb. Then the wide open of social media. Now back to specific again. A generation from now it will shift again.
(Not to say that there aren't a large number of great projects off the main path.)
> It's a cycle
Lrf, guvf vf n cha.
And here with the birth of broadband I thought we'd moved past the age of line noise.
$ echo Lrf, guvf vf n cha. | rot13
Yes, this is a pun.If you could somehow solve 2, 3 and 4, I assume there'd be many thriving topical communities in parallel, just like the forums of yore that you speak of.