The T39CMB is the oldest MB (excluding Stacks), already more than 7 years old. The first visible topic was posted in 2011. The people there are generally more caring/accepting, than fighting or using bad language, but there has been the Defender War on there.
The T39CMB has a lot of members including Zoey, Maya, Star, Charity, Lani, Lillian, Mickey, Micky, Lucy, Sassy, Cara, Ocean, and more. They tend to do a lot more RPing than other things, but they have some poll threads and similar things.
They don't have many MBer x MBer ships besides Lyzzy and another one that I know of, they're more shipped with book/TV show characters.
The MBer who's been on the longest is Rad, who's been there since May of 2012 and he has 178 pages of posts, but Taylor has around 348 and she's been there since October of 2012. (She mainly is on chat; mostly on the MB nowadays she RPS and says bump.)
One of their most famous trademarks are what I nickname has "Adjective-condiments" (Eg: Awesomesauce, Coolsyrup, and Epicsprinkle.), and Lyzzy (Lyssy X Oz). Oh, and disembodied hand brownies. Don't forget those.
A common thing you'll find on 39MB is "what I imagine you to look like" threads. Don't mind them.