Why not make an open soure alternative? How much do you hear about blogger.com and livejournal after Wordpress came out? How much do you hear about AOL after the Web came out?
They've already tried. But the biggest thing about social networks is that they really only work if the people you want to be connected to are there. Diaspora was an open source social network, and it went nowhere. Ello was supposed to be a social network that didn't sell your data, and they ended up not being able to make it as a social network. They're still around, but they've pivoted a little.
Because there are open source alternatives. Nobody uses them. Nobody used google plus, and that was a realistic replacement.
Oh yeah? Please list the top 3 alternatives which are open source.