The dutch social network Hyves introduced this a few years ago, and set it on by default. They got a LOT of comments over this (which they handled quite well: sending around flowers and all). But within a day it reverted to off by default. You can still switch it on, so others see your visit's, but I think it's not that common.
Power users (paying) do get some aggregate stats tho (last month, 40 people, of which 54% girls, visited your profile.)
