I don't follow. I can post the exact same listing on FBM and CL, and with CL I get bombarded with scammers, and with FBM I never do. In fact, with FB, I'm not sure how you would ever get contacted by scammers at all, since it's tied to your FB account, so someone would have to go to the trouble of maintaining a fake FB account just for this purpose, and it would presumably be quickly flagged and disabled as soon as a couple people got obvious scam communications from it. For better or worse, the "real person" feature about FB and lack of privacy, actually seems to work in its favor here. Anyone with a phone (or who can spoof a phone number in your area code from Nigeria) can contact you on a CL listing, whereas with FB the bar is much, much higher and there's a lot more policing.
