I think it's rude to add yourself to someone's friend list without introducing yourself. I don't care if your introductary note is something as lame as "hey add me back" or if it's a short personal biography. Heck, even a "I like anime, you like anime, woo-hoo let's be friends!" accomplishes the purpose of letting me know what you're doing on my friends list.
I think it's creepy when people keep adding me and I don't have a clue who they are. Not a CLUE. Never seen these people in my LIFE. Then they don't say anything. They're just there, theoretically reading my entries (because that's what you do with your friends list, right?) and getting to know me and I don't know a blink about them.
Yeah. CREEPY PEOPLE!
So, y'know, if you add yourself to someone's friends list, take that extra five minutes to introduce yourself.
Maybe they'll FRIEND YOU BACK if you do that.