I hate to be a pessimist but it's gonna be damn near impossible. It's like knowing a band by a few promo pics as opposed to songs and band name (trust me I speak from experience).
I'd say your best bet is to try googling a comment they left? If you remember any of them. It would probably have to be word for word though. Or if they were on an unpopular club you can go there and try to find them. Other than that I don't really know of a way.
You can't A person's ign is their calling card if you can't figure that out. You can't figure out which username your friend has. It's like going into a neighborhood and forgetting what address your friend has. No identification no pinpoint shot to finding said friend..
