In order for smaller platforms to really make a dent in the social media space they will have to gain serious momentum. Facebook has such a huge gravitational well in the social media space that content creators are at best doubling up with Minds, Steemit, etc. An0maly said in one of his streams that he has so many followers on Facebook that leaving the platform would hurt him too much, and he'd rather "fight from within" to change their policies of censorship and creating echo chambers for people than to leave for a more open and free platform.
If any small platform gets big enough to start to draw users completely away from Facebook, Twitter, whoever, they have such huge war chests that they can swallow up their competition before feeling any loss of revenue. It wouldn't make any sense for them not to. The question I don't think is if the monster networks will buy out the small ones, but if they will impose their branding, community guidelines and other policies on them or embrace the differences and let the other platforms exist pretty much as-is.
The only alternative that might happen is the small platforms band together as some kind of collective or cyber republic, share traffic and content, govern collectively and commit to not selling out to the digital superpowers. However, the chances of something like that becoming reality are less than slim.