It's not free, but it's pretty cheap. When you don't have to have massive marketing departments, huge management bonuses, expensive office space, crappy proprietary software, and massively scaled highly available platforms it costs a lot less to run a social media platform. Donations can often cover it all.
If one server gets too big, they can just cap registrations and people move to a different server.
Yes, even the UK had to accept this when they were in the EU. But unfortunately it wasn't very proportional, due to the small districts. Plus no one paid attention to EU elections, so it didn't have much impact on politics (e.g. it didn't help win any arguments about changing voting systems).