I'm thinking about coming down for Memorial day weekend and I'm just wondering how crowded it usually gets during this time. Thanks!!

We're Florida's Florida, so I'm betting it will be pretty busy. (It will be my first Memorial Day here.) If you haven't made reservations yet, I'd be quick about it.
My wife and I have been going down to Marathon over Memorial Day for the past 4 years. Memorial Day is a big time in the Keys. Not so much for partying, but to get out on the water and relax for a long weekend.
The traffic down on Friday will be something else. If you can, plan on getting in on Thursday. DO NOT try and get back to the mainland on Monday. The first year we traveled on Monday. It took us about 6 1/2 hours to get from Marathon to Miami. About 4 hours longer than it should have taken us.
If you are flying in and out of Key West things will be much easier for you..
Have fun