My experience over the last 10 years or so is that most of them are detrimental, if anything, to the longterm success of a community site - IF the paid posting services are used as the sole source of community building.
It will always require serious owner and moderator participation... IE, plenty of blood, sweat, tears and multiple poster identities
and a bit of luck.
Rob
p.s. all to wind up with mediocre CTR and eCPM at best hehehe.