I have had to name change for this as 'they' might be on here...
if it's a good friend, give them a call/text and say 'hey, whattsup? have I upset you' - when this happened to me I explained it was ok if they didn't want to be facebook friends but it was really important that we were actual real life friends. To be honest the FB thing did upset me but rationally it was far more important to me that in real life we were still mates. We are now both real life and FB friends again :)
If it's someone you are not too bothered about, or who has form for this kind of thing then Just smile and Wave.
This happened to me with a family member. Do not mention it to them. Put on your best smile when you see them, greet them pleasantly and act like nothing happened.
When this happened to me with a family member I waited a few hours and then I messaged them saying that I hoped I hadn't upset them and that I wold never want to unfriend family on FB. I sent a friend request along with the message. Then when they responded negatively I thought to myself 'ok then', withdrew the friend request and life is much calmer.
We did however have to meet up not long after so I did the whole smile and wave thing.
Ultimately it is 'Just facebook' and you will know how much of this is a spur of the moment fit of pique or if there's history behind it so trust your instinct.
I will just say life is a lot calmer without the constant drama of people who use Social networks to be passive aggressive and meddling.
NB - it sounds like they have just quietly unfriended you and you've only just noticed?