If Facebook has one standout application it has to be Photos. Measured on its own, it is the largest photo site on the Web. A full 69 percent of Facebook's monthly visitors worldwide either look at or upload photos, based on comScore data. And more than 10 billion photos have been uploaded to the site. And it's been pulling away from its competitors. As can be seen in the comScore chart above, as recently as last September the top three photo sites in the U.S. were running neck-and-neck, with Facebook Photos at 23.9 million unique visitors, followed by Photobucket at 21.3 million uniques, and Flickr at 19.5 million uniques. But by January, the number of monthly U.S. visitors going to Facebook Photos shot up 41 percent to 33.6 million. Meanwhile, Photobucket is up only 7 percent to 22.8 million, while Flickr is up 12 percent to 21.9 million. (Picasa is a distant fourth in the U.S. with 8.1 million). In other words, Facebook increased the gap between its closest competitor (Photobucket in the U.S.) from 2.6 million monthly unique visitors to 10.8 million. On a worldwide basis, the gap between Facebook Photos and Flickr (which is the No. 2 site globally, and looks like it is about to pass Photobucket in the U.S.) went from 41.2 million unique monthly visitors in September to 87 million in December (the most recent data available, see chart below). What accounts for Facebook's advantage in the photo department?