So who stays online when the grid goes down?
The grid isn't totally out, it's just not accepting new log-ins and anyone left in world is unable to teleport from sim to sim. The individual sim servers are up and running, it's just the network that links them together that's borked.
People who run bots and many campers use text-based viewers that keep the avatar online, able to receive IM's, but usually sitting in one spot, this segment of accounts is largely unaffected by grid outages.
People often wonder just how many bots and campers and other forms of non-live avatars are active in SecondLife, that figure of 15,000 users still on the grid when the grid goes down is probably a pretty close approximation. That would suggest that at non-peak times, something like 1/5 of the grid is bots, which is pretty close to the observations and sampling figures people have come up with.
It would be kind of creepy to be in-world at times like these. The only other people you find are frozen and unresponsive, like something from an episode of