In local community FB posts I'm seeing replies from people with usernames like DaringAlligator2732 and ConcernedHomeowner14048. These don't seem like the "real identity" facebook usernames; they kind of feel like anonymous or sometimes even randomly generated names. How and why does that happen? Has it been happening for a long time?

Bots.
Need to drive that engagement now that fb has pretty much hit the limit on organic user growth.

Apparently a new twist where you can be anonymous, because Facebook just wasn't cancer enough as it was

Pretty sure this is just a new way of anonymous posting. I run my local FB community page and it recently asked me if I wanted to allow "nicknames" in the group. Assuming that's what these are it just allows people another way to not associate their real name with what they are saying.

That’s an option for people to post/reply anonymously. It has its uses, like if you want to ask an embarrassing question on a public group. But as anything else. obviously it can be abused.

looks like the exact same system that Reddit used for their trashy usernames too

wouldn't be surprised if it's all the same database

same database

Same agency.

https://www.nytimes.com/2015/06/07/magazine/the-agency.html

To be fair all the real usernames have dried up on sites that have like a billion users. How the fuck am I supposed to register a new account on a website when every possible permutation of anything I would actually want as a username is already claimed?