Thanks for your input. I see the viral point if there is an auto wall post, but if user has to approve wall post, unless there is an incentive (like in game app when user tweet or FB post, they would get golds or coins for the game), I do not see why users would post in their wall unless they are truly a fan to the app/site/marketplace.

I am asking this question as I am trying to decide if I should implement FB connect to the marketplace startup that I am working on. If I do it, it would be for

sign up / registration

login

wall post when user post a listing

wall post when user buy a listing

the only thing that I find it a value is the wall post, but then if user does not authorize it, then it's useless nonetheless (unless I give incentive to post).