How does Facebook, etc make the web less open?
Many of the things Facebook is used for were originally disparate services-- think of the smaller social groups that would have been mailing lists, or at least message boards. Event management-- RSVPing and messaging participants-- used to be a fairly competitive space too.
If you need services in those spaces, and lack the technical skills to roll everything from scratch-- but it doesn't fit with the Facebook model or business tastes-- you've got far fewer options than you had 10 years ago.
I'm thinking, for example, a community I enjoyed many years ago. It was for collaborative writing so most everyone posted under pen names. In 1998 it was implemented by a mailing list which got cross-posted to a newsgroup. In 2005 it would have probably been thrown up on a cheap "hosted forum" service like ProBoards. In 2018 it would be a Facebook group, and almost certainly stuck with posting under real names. And that's the most benign "we lost something" outcome I can picture.
And this is all distinct from the very serious free speech (not first amendment) issues, which are another huge problem.
I wish instead of freaking out over the latest petty outrage du jour, our societies would spend more time considering whether whether we might be going down the wrong path in a variety of ways. All the same stuff we see on the news day after day after day almost feels like cover fire.
Why are you asking such an obvious question?
I happen to think whether they should be allowed to do this considering their size is worthy of a public discussion, but it doesn't affect the overall internet, at least how I think of it.