Ranks are automatically distributed based on... reputation if I'm remembering correctly. As you gain Rep you'll level up in ranks. Reputation being earned primarily by getting a reaction to one of your posts. Looks like there's "Thanks","Laugh", "Sad", "Confused", and "Like". I believe they all count the same, but I'd personally steer away from being *that* guy that only gets sad/confused responses from people. The responses can be on any post btw, be it a file, photo album, status blerb, etc. In addition if you get the most likes/responses in a single day you will "win the day" which just ticks up a number on your profile and the last day you won will also be displayed on there.