The abuse and smear campaign I went through with the abusive ex really showed me who was my friend and who was a user. I now only have real friends.
The rest I nip in the bud or keep at arm's length as acquaintances. You can tell them because they always do an early boundary-push. They don't want to waste their time faking friendship with someone like me or Bluntness who won't stand for their crap.
That is why you feel they were nice at first - you missed the boundary-push. The slightly-too-imposing-for-someone-you-barely-know request. The type of thing that you see people advising every day on Mumsnet, to be 'nice' - 'just take their child every week, it's the nice thing to do', 'just let her have the money if you can afford it', 'just babysit this once, she's desperate', 'oh, he's probably depressed/autistic/ADHD - give him a chance'. Not every poster, but I bet these are the same people who are troubled by these user friends.
If you are unclear whether something is a boundary-push (and it's easy when you are used to it - it's a discomfort feeling) - say yes, and immediately ask for a reciprocal favour - 'oh, yes, I'll run you to the station an hour away, that's really good because I needed someone to take me to the airport next week - can you do that?' You'll find the favour is not forthcoming on their side even if they do agree.
To be clear - I will go the extra mile for my real friends. But acquaintances? Only if it suits me and only if the request is proportionate.
When your life isn't full of users, then there is room for the genuine people to be in it.