If those 5000 members are active, then the group can be effective but it's more beneficial for personal earnings and if people keep posting links to new sites, the members would get bored and leave the group as it's as good as spamming your referral links. Only those genuinely interested in the sites would join or if you offer some incentive. There are special facebook groups that offer members incentives for signups and they are more effective than the other groups. Some are just lucky enough to get 100s of referrals as their blogs or groups have active members who willingly join those sites.
i believe so since people who mostly stay on facebooks are newbies who's been courious around to see whats new and appealing for them to earn profit, and facebook posting is totally can earn great result for those site who wants to advertise since facebook got huge community number where people are interacting to each other, forget about those spammy post their since somehow it can help to get a little attention to adventurous persons online.