That possibility is there where dolphin sites can grow to a masive extent...it's all in how big you want it and how well you promote it.
The ability, functionality and tools are there, some of us need to know how to use it to it's full extent.
Dolphin 7 is still new and i believe it will grow much much more and who knows...at a later time we just might see a nice load of successfull dolphin sites.
All dolphin sites i'm sure aren't listed on Boonex so who knows how many successfull ones are out see more there.
forsakencherry.com was running pretty successfully
myviewin.com looked to be running successfully

@cal - I also agree that there's not many sites listed that are running a high ammount of members to apply such a feature (groups alert approval)
I'm sure alot of us think that such features wont be popular or big but preparation is the best way to go cause the question is...What If???

@ladybug - i also agree with the fact that one site could have multiple group types as using artistry social site as an example.