^^you have to maintain a specified ratio (I believe above 1.0) for a certain amount of time before you get invites.
And yeah they use an invite system to get rid of leechers and its also qauility control. Invite sites are always the best.
Some sites also have a max amount of members, like lets say once they hit 50,000 members its a closed site. The only way you can get in is if someones account expires (because of lack of activity), get banned or whatever.
I had like 12 invites at oink or so and gave them all away and my ratio has forever been right around .87 to .90 for the longest time.
Filemp3 is pretty tight though, if you check rap and hiphop and do a search you'll see a bunch of shit you never heard of before, real underground shit. I even see some siccness cats on there right now, XianX to name one.