I don't like it when I get tagged or mentioned in the comments of a random article or advert. Presumably so I will see it, find it funny, or take it as a massive birthday present hint. Annoying too when you want to read comments but it is just a list of people's names, with a few emoticons at best.
That and competitions where you have to comment or share their company's picture to win a meal out. One was suggesting they were giving away 25 motorhomes.
Probably the top thing that makes me want to leave Facebook. It is worse than chain emails and spam IMO.