I'm curious about these billboards. I clicked the "create ad" link to check it out and it seems I can make a billboard for any personal reason? Most I see right now are just links for people to get ToT visitors. So does that mean I can make a banner and just have it like, "Look at me! Be my friend. ILU." sort of thing?

pretty much, yes. Though you should know that all billboards have to be approved by a staff member so if you do make something unacceptable for whatever reason, it will be rejected. However, if it is rejected your csc will be returned.