1. Tagging in unrelated images. I'm sure you've been a victim of this one. You see a notification that you've been tagged in a post, and when you take a look it's an image with a motivational cliche or an advert for an event.
2. An invitation to play an online game. If you're the type that enjoys Candy Crush, the chances are you are already playing it. Constant requests to join people you don't know playing a game you don't care for are pointless.
3. A personal attack. However strongly you feel about an issue, there's no point being rude to someone who disagrees (even if they started it). Just walk away, in a virtual manner.
4. One-track posts. Posting on one theme all the time, such as motivational quotes, is dull in the extreme.
5. Telling people what to do. Advice is fine, since people can choose to take it or ignore it. Instruction is quite different. Telling someone that they simply must take the course of action you suggest is arrogant and rude.
6. Sending personal messages to strangers. There's an etiquette here. If you haven't had any connection with someone, don't send them a PM.
7. Adding people to groups. This is incredibly rude. Adding people to your group without asking first is like online kidnapping. Don't do it.
So there you go. On the other hand, if you are polite, helpful and respectful of the views of others, you'll have plenty of friends.