Yes, it is possible. I think it will be something like this:

image source

where proprietary software will eventually be replaced by FOSS software. it just takes a while (Linux was released in 1991).

also, for social media, it's not so much about the software used, more about who controls it, and hosting plays a significant part in this. the question is, how do we put up an organization large enough to actually sustain that many users?

who pays for image/short video upload for a billion people? small instances on the fediverse already cost real money. feddit.org has 1000 users and reportedly already costs $1000/month to host, IIRC (which seems expensive, even to me, anyways), and catbox.moe, which is a donation-funded service also costs around $1000 (says so on their website). that number would obviously increase sharply if there were more users. So: who pays for it?