Mostly I pruned multiple service accounts for people I only talk to in one place. A surprising number of my friends had three or four old IM names from their pasts, some of them five years gone.
A few of the nicknames had been used once because of one incident: a temporary breakup, avoidance of one person or another, a joke. Some were from friends' previous jobs.
Two of them were dead. The collision of instant messenging and death was unexpected.
None of them were people I don't or shouldn't or won't talk to any more, which was a relief.
And then I left a few in because I could not remember what they meant. Accidents? One-time work contacts? A stealth name of someone I forget? Keep it in there, I guess.