I use my customs mostly as the alternative to AIM. I *hate* AIM and don't like using it. I get a desktop notification of new email with gmail and can respond pretty instantaneously when I'm online, without trying to be all cute with a username and all that. Now, there are some that may wanna mix a scene into their customs but I've never done that myself, and just set up new posts for it.
That said, I also use them to gif spam, lyric spam or just kinda keep a contact going with characters in some way shape or form. :)