Neil Gaiman is on Tumblr, and is very active there. Also one of the only celebrities whose presence is actually enjoyed, not merely tolerated. Also present are Diane Duane and Peter Morwood, and likely several others I haven't found yet. They all interact with followers and fans alike, and are happy to receive messages about their works.

This is not a hard-and-fast rule, but rather a general guide: most creators are happy to receive fanmail by any means, hand-written, typed, electronic, you name it. Just don't be weird or creepy.