The photo site MyMemory.com conducted a survey of 1500 women who use Facebook and found that one in four admitted to deliberately posting unflattering photos of their friends.
From Gawker:

Of those responding positively to the question, one in three claimed they were doing so out of retribution for a similar slight made against them. A majority of the rest said they had fallen out with the friend in the photo.

So, you know, maybe keep an eye on what kind of company you keep because the world is swimming with jerks. Over in England, on the other hand, the world is swimming with drunks:

An earlier survey conducted by the site found that Brits are drunk in the vast majority of the photos that appear on Facebook. Some 76% of nearly 1,800 Facebook users polled said they were at one stage or another of inebriation in most of the photos in which they were tagged.