Just have a look at Google+, millions of people gave that a try but in the end only a very limited number of those formed enough connections to justify primarily sharing things on there.
The question then really is, if there is enough of these niche groups that will provide enough user retention to make the economics work. There has been plenty of things written regarding app development and the difficulties in supporting recurring costs with one time fees. I suspect you would need a pretty significant userbase to support even a small team on cheap one time feature purchases.
This was a pretty connected set of early adopters moving at once. Would things have been different if Google had quickly relented and said "okay, sure, if you wanna be Fuzzybutt Wolftaur on G+, go for it"? I dunno. But chasing them away, along with all the other people with scarier reasons to want to avoid using their real name, sure didn't help.
But I do know a bunch of people who value their privacy. They just don't know what they can do about it.