Now this is a bit surprising. Dusan Writer is reporting the purging of about 1 million Second Life accounts (determined through statistics and changes to in-world search results).

I wonder if they used the Devistating Tactics and Cold, Monkey Efficiency of the Monkey Assassin Organization for the purging?

These are most likely inactive or unused accounts. There is no official word from Linden Lab. This is a relatively small number of total registered accounts and can be chalked up to cost saving or reducing the overall size of a database (which is always a good idea). General maintenance if you will. Dusan claims that almost 14,000 Second Life accounts are created per day so the purged amount should be recovered in a relatively short amount of time.