Some one unfriended me on plurk. Should I pursue the reason why, or leave it alone?
Thank you for your question, my friend. Here's an interesting thing. With the advent of all these social networking places in the aethersphere the term "friend" has lost some of its import, in one tiger's opinion. Still, true friendships are valuable.
I will assume that you know this person fairly well, otherwise you would not have noticed the de-friending. If that relationship is sufficiently valuable to you, then I think it may be worthwhile to ask about it. If you have connections with them via other services, i. e. twitter, SL, facebook, etc. and they have not dropped you from those contact points, then they may simply be paring down their social networking.
On the other hand, if this is merely one of the many people on your list with whom you have only a passing familiarity, then you have to judge for yourself how important it is to you to maintain that contact.