It is the one advantage of being an only child - actually, one of many, but this is a good one - you can always create a little world in your head. I don't think I ever had an actual imaginary friend, but my teddy-bears and bunnies had personalities as well as names... the only reason they didn't talk out loud was because they were shy.

Hate to break this to you, but this particular phenomenon doesn't just apply to only children. I think it's just a normal part of being a kid. On the other hand, it could be an early sign of madness and I'm just in denial ;)