I've been noticing a lot of people (mostly in the erp forum) who either can't figure out or don't even know direct messages are possible so I wanted to give a little tutorial.
On mobile it's slightly hidden, but there's two ways to easily get to the messages feature. First one is to simply turn your phone sideways and you'll se a bunch of options show up that were previously hidden. Or, you can click on the menu button on the upper right (three horizontal lines) to access those same options.
For direct messages, all you have to do is click on the username of the person you want to message, then use either of the options I described above to find the "send message" button.
Hope this helps.