I see two points in your argument:
Everything becoming a social network
People working at tech companies have to justify their salary somehow and this is low hanging fruit for adding 'features' as all people feel some need for connection. Feeling that a place is alive with other people will motivate your more to engage with it, rather than say, your own Git hosted server. I don't mind the social features added to GitHub as long as they don't take the main stage, like it did in the LinkedIn transformation.
GitHub monopoly of open source
GitHub has for most of the time been the main place for open source. I don't see a monopoly as necessarily bad as long as it remains focused on some values other than profit. I would rather have one big Wikipedia than a shitload of small fractured Wikipedias. Can it become a problem going forward, like it did with Reddit? Definitely, but I am cautiously optimistic. And in the worst case, git is heavily decentralized by design so you're one git remote add && git push away from moving. Migrating issues would be a bit more of a hassle, but surely there are solutions. And CI is not easily portable, but not a huge amount of work to convert to other formats.