There's nothing wrong with using Gmail for email hosting if you have your own domain name, it's easy to then switch over. But if you use an @gmail.com email address and Google decides to ban you, you're screwed.
Same story with Facebook, there are quite a few people that I would possibly never be able to contact again if Facebook banned me or them. This isn't a new problem caused by Facebook though, in previous years there was the same problem with email, and before that, if someone changed their address or phone number you'd lose contact. There are actually people who I lost contact with who I regained contact through Facebook.
Anyway, my point is that centralisation isn't bad if it's painless to decentralise again. Git and Github are fine, at work we could switch to a self hosted solution or Bitbucket in a fairly trivial amount of time. Same for our work emails, we could switch form Gmail to another host with fairly minimal disruption. Centralisation with locked ecosystems, like Facebook, are not good.