If you think a Twitter Fav is the same thing as a Facebook Like, think again. Favs have a plethora of different meanings depending on context and how annoyed you feel at any given moment. Genres range from "damn, you fine" to "I need a passive-aggressive way to acknowledge how stupid that was." Here is a guide to the different types of Twitter favs. Read it, study it, print it out and keep it in your wallet: only then may you know the true meaning of the fav.
Deployed almost exclusively on personal tweets about your undateability or selfies where your hair looks good. Also applies to people who fav any and all things you tweet, even if they are banal/stupid/something you're going to delete in the next five minutes.
"When a tweet annoys him, he does what he calls 'hate-favoriting,' a sort of sarcastic use of the favorite option. 'When someone is totally awful, hitting 'favorite' is the most perverse thing you can do.'"
A relative of the hate fav, but with a little bit more irony.
The "I Agree With Your Ideology" Fav
Deployed for politicians tweeting opinions you agree with or celebrities hate-tweeting Woody Allen.
When you favorite a subtweet about you, particularly if it's to make the person aware and even nervous that you know they are subtweeting you.
The "Do Go On" Fav
Favoriting a tweet in which someone said something nice about you.
"You Are Brave" Fav
Faving the tweet of someone who said something you were thinking but were too afraid to say.
Sick of Your Schtick Fav
Only for the truly sadistic, some people use favorites as a way of saving links they want to read later.
The You're Clever, But Fav
Of course, always remember: