FTW - Not an anti-establishment chant of Fuck The World as I originally thought, this is actually an abbreviated way of saying For The Win. Which makes a lot more sense considering the context I've usually seen it in - "Fresh cookies FTW!"
Reference: Urban Dictionary
I Drink Your Milkshake. This sounded like some depraved sex act and seeing Jason Calcanis constantly squealing on twitter about drinking someone's milkshake only reinforced this. It's actually a scene from the movie There will Be Blood.
Reference: YouTube.
Locats/I Can Haz - I guess I get it, but I don't understand the fascination.
Reference: http://icanhascheezburger.com/
FAIL - Same general idea as I Can Haz, take a picture that's already funny, throw FAIL on it, laugh, repeat. Or find an idea or event that you'd like to discuss in a thoughtful matter, summarize it in about 135 characters, append FAIL, chortle, call your parents to brag about conquering the internet, repeat.
Reference: The FAIL Blog.
First - The combination of having nothing useful to say and no meaningful existence that drives someone to constantly refresh the homepage of a highly trafficked site and post a comment containing only FIRST11!!!! as soon as a new article is posted. This is usually followed by 2 or 3 more comments also saying FRSIT! or some variant thereof, and then half a dozen other people condemning them for being idiots.
Reference: PerezHilton
See Also: eBay Feedback