So I hastily changed the password then deleted the account. I hope it's gone and I don't think anyone was really following me, but I now feel slightly embarrassed that I hadn't kept a closer eye on it.
Also, just to be on the safe side, I've just been and changed the password on several other social networking sites, including LJ and DW. Yes, I know you shouldn't re-use the same password, but I had because otherwise you have so many you just can't remember them.