I had a FB for abut a year, a good friend who lives about 2 hours away.... its now evolved into a monogomous relationship and has been like that for about 6 months. I am very happy.
For us, it really worked, as when we first started we had both recently come out of relationships and needed a bit of time to have a bit of freedom and fun with others.
To be honest, I wanted to have sex, but didn't want to pick up some random guy in a pub and didn't want to complicate things with someone at work - so a friend who I trusted seemed a good idea.
For me, I realised how I felt for him (and him for me) when I had a car accident and he dropped everything and drove for 6 hours straight to come and see me at hospital (I was visiting family at the time). I liked the way it felt to have someone 'there' for me and realised I'd do anything for him too. Oh, and I didn't like the though of him doing anything with any other girls! Took me a bit of time to admit this bit to myself though!
For us, it was quite organic, and a full blown relationship would not have suited either of us at the beginning.
However, there have been a couple of arguments along the way, with neither of us really wanting to say what we did/didn't want and the process of getting from FBs to boyf/girlf was a little bumpy, to say the least.
Another story for you. I hope that helps. What is your story? How long you been single? how long have you known him? How do you know him?