Craigslist: I met some nice people interested in old bikes, but it was 50/50 between people who would show and flakes who just were no-shows. For every person who would show and actually take the time to look at and maybe buy a bike, there was one other person who would talk a good game, then never show up.
Facebook: Lots and lots of messages about trading for other bikes, usually a damaged big box bike. For whatever reason, Facebook draws quite a few people who have this low-ball/trade hybrid method of trying to deal with you. You make it clear you are selling for a fixed price, then they either offer you 10% of your price, or they say they have a nice Wal-Mart mountain bike that just needs a little work for trade. Some of them have sketchy, clearly fake user names and profiles. Give me a break. Then there are the talkers who never show, or who use the generic "Is it available?" but nothing after that.
eBay: The buyers here are a little more serious usually, but there are scammers on eBay too. Getting expensive to sell on eBay, and their red tape is making it harder.
Honestly, the CABE and BikeForums have been the best for me. It's a self-selecting group for the most part. I will say I've had flakes and BS artists on this site, but it's not nearly as bad as elsewhere.