I think it will just take a week or so for people to trickle back in. The PM system coming back up and the counters and the emailing systems will all help, once they're running once more. 
I know what you mean, though. I have some RPs that have been going for over a year and one of them, I am fairly sure only got on when they received an email to say to get on - so with that system gone, it's highly unlikely they'll return. :(