Would this be socially inappropriate (see details)?

I suck at judging these things.

I am taking ¾ of a year away from college to get surgery. During my first quarter I made some good friends and I want to stay in touch with them in the hopes that we can just pick up where we left off when I get back, but I’ve been doing a really crappy job of that especially as of late. I haven’t talked to anyone from school in maybe a month.
Yesterday I found out that my best friend from college has had a girlfriend for about a month. I had developed a pretty big crush on him, so the news was rather upsetting. Within the past month I have made one post to his facebook wall that could be considered semi-flirtacious in my own awkward way, not knowing that he had a girlfriend. So, I’m probably not her favorite person in the world. I have no idea if she’s the jealous type or anything about her.
Lately I’ve been in quite the self-pitying slump and haven’t thought much about trying to stay in contact with my friends. Learning this news about my friend, while disappointing, kind of reminded me about my group of friends and how much I miss them. I want to get the lines of communication going again, and to do that I’d like to write my best friend a letter and send it to him in the mail, just updating him on my life and asking him about his. 100% innocent friendliness.
The last thing I want to do is make him or his girlfriend think I’m trying to sabotage their relationship. But the second to last thing I want is for him to forget about even being friends with me now that he has a girlfriend. I feel very awkward about the whole thing.