Like it or not, the general public uses fb/google or wechat, maybe, but only maybe, read a few news site, do banking, even less buy things on amazon, which, out of US, UK and DE rather nonexistent, and that's it.
Think outside of the US, please.
Literally half the people with internet connectivity do not use Facebook at all, much less exclusively.
I probably read it like that because of his following paragraph, which talks about how particular websites have this "walled garden" feel and how some people don't move much away from them to the point that they consider these particular websites to be the whole internet. Since the topic of this subthread is about how people of the internet have less in common now that it's become mainstream, that interpretation seemed to contribute better by raising the point that, to many people whose interpretation of the internet is a "walled garden" like Facebook, their "internet" has not become mainstream, or at least not on the level of the "internet data connection" or the real internet.
He specifically says that using the internet for blogs and websites (i.e. things other than Facebook) is not "mainstream". Maybe the Facebook comment was not intended to say that's what the "mainstream" use case is, but that's how it reads, and I'm frankly not sure what mainstream usage would be if it's not "websites".