[lxgr]

At least in my group of friends (which is probably a very bad sample), hiding "last seen" is very common.

I really dearly wish there was one for the online status as well, and I'm glad to have spent my teenage years in a time where SMS was the norm, where neither read receipts nor online status was available. It made for a much more relaxed messaging experience (which iMessage seems to be much better at approximating).