Of course, Tumblr has to be the cool kid. Tumblr finally rolled out a new messaging feature, but you have to be invited by one of the lucky 1,500 users they exclusively rolled it out to in order to use it.
Previously, you could message through the ask box on Tumblr, but it was kind of a pain and didn't have a great way or organizing current or old messages. This new messaging feature will be in a threaded format, like most typical messaging functionalities, making it easy to connect with your fav bloggers.
Don't worry, if you or one of your friends aren't the lucky ones invited to try the new feature, Tumblr promises it will be available for everyone soon. Tumblr's doing this soft roll-out to make sure its "servers can handle" this new functionality across the entire user-base. It has nothing to do with Tumblr being the coolest social platform on the interwebs.