I've met a few people on this board and an unrelated one that I consider friends. I think it is different like Spiderlashes said, the chemistry element is not an issue in friendships. I think another difference is that I and most people I know don't go on message boards to hook up with people or to sell themselves to anyone, so most people (and yeah, there are troll/alias exceptions) tend to represent themselves in a pretty straightforward manner. So when relationships develop it tends to be in a pretty normal, organic way.
I will say that even with my platonic friends when I met them IRL, it took a little while to get used to how they look and how they sound because my mentla picture was always a little different. It did not impact the friendship, but I could see how it would totally impact a romantic/sexual relationship
To Trenell, MizKerri and geeky:
I pray none of you ever has to live in a communist state.
Geeky is my hero. She's the true badass. The badass who doesn't even need to be a badass. There aren't enough O's in cool to describe her.