The beauty of the Internet has always been the way sub-nieches grows into their own communities once their parents become too diverse. For me, Facebook always felt like the last monolith that "tried to do it all". It now acquires sub-niches instead.
Don't ask "where are the things that look exactly like X social media platform but with different details" (e.g. facebook versus google+) instead ask "where are people spending their social time these days, is it in the same places it's always been or are new places becoming more popular?".
Also, to add on to your point about "sub-niches" one thing that facebook lacks is character, even though one of the primary organizing principles for social spaces is unique character and shared interest (look at HN, for example). People organize socially around interests. One easy way to take on a monolith like facebook is to take a stand, carve out a niche, have a unique character and build a sub-niche there.
she's aware of the niche sites and is writing about large general interest sites.