Ok several! This is for entertainment purposes only and has no basis of fact whatsoever.
1- Most likely:
The class is currently full. As people have turned down acceptances, UAB has sent them out (e.g. last week there were a couple, I think); we shot right through the first part of the WL and that has probably taken care of most of the drops up until May 15. Acceptances will continue to trickle, and we most likely won't see the end of the middle tier. Keep in mind, a majority of drops might have occured, as the first tier must have been around 40 people...and seeing as we are in the second tier, there have possibly been close to 50.
2- Facebook theory: I'm not really into facebook, but even I would probably eagerly join the UASOM 2014 group if I were accepted. The fact that the group is only at 80 now is a bit baffling, given a projected class size of ~175. There is a [remote] chance that many more people than expected have turned down acceptances, and UAB is currently scrambling to prepare to release a large group of waitlisters to get the class back into an acceptable range for this date. The lull in acceptances could be explained as some kind of damage-control/ mass reallocation of acceptances that will soon occur
3- "No one cares about may 15 deadline" theory- I see this as likely. It would explain the lull in acceptances (class is full- no need to offer more) and the low FB group numbers (people aren't sure they want to go to UAB yet-need more time to decide, therefore didn't join group). While UAB is an awesome school, it is expensive for OOSers, and it may be kind of a backup choice for them. They may be holding onto acceptances, but not really taking it seriously (therefore, no FB group), and waiting for their instate instiutions or dream schools to come through.