We almost bought a house through a FB selling group, and I know a few people locally also did the same - essentially saying "this is what I'm looking for, does anyone know anything available". I know two people where it worked very well - they saved the estate agency fees.
The house we ended up buying was through Purple Bricks. I cannot shout enough that they were HORRENDOUS to deal with as a buyer. They were unprofessional, lazy, didn't know anything - at one point I resorted to emailing them to ask if they were actively trying to cause the entire chain to collapse and their response was to genuinely ask me what the address was of the property that THEY were marketing. I cannot stress enough how awful they are to deal with.
We had so many problems with the purchase because there was no one there to communicate with us at all. Literally the only thing that kept me sane was the thought that our vendors had been motivated enough to instruct an EA, therefore, they must have wanted to move. I think if we had had the same issues and it had been through the FB group via a private seller, I would have queried whether the seller genuinely wanted to move because they hadn't even been bothered to instruct an EA....
My EA was crap at selling my house, but their after sales support was phenomenal. They constantly kept our buyer updated, they constantly chased Purple Bricks to try and get some movement, and they did everything possible to ensure that the chain remained in tact. Based on my experience, EAs should do more PR around the fact that you pay them for their post-sales activity, not just to be listed on RightMove