i love to use ym i dont like fb ,fb for only knowing people ,and then u have good moments there,but now-a-days i want to quite all those social sites cause its good for nothing which i realize after one decade lolz
well i do use yahoo mail everyday but not the yahoo messenger. So if I had to choose between yahoo messenger and facebook, my vote would go to facebook.
Facebook is better. your friends are all there and you can easily share photos and videos. and you can even video call. you can even have a group chat. almost everything that you need is in facebook.