If you want to be mean to someone anonymously, there are lots of ways to do this online, but few provide assurances that your target will actually see what you’ve said about them. So you risk wasting that nasty little witticism . . . unless the person is on Twitter, that is.
One can be fairly certain an @mention will be seen on Twitter – and appropriately sting (just like a passive-agressive blog comment)!
And yes, even when you’re tweeting to celebrities, they see them. Well, THESE celebrities do, at least.