Perhaps because they are continually rolling out new products and split tests.Perhaps because they are terrific product marketers and designers. And/or perhaps because they need to figure out how to message 600m+ users in efficient, effective ways. ... Facebook is the best at "in the river marketing". Yet another example:
This week Facebook rolled out a new Facebook Photos features to all users. Rather than an email announcement, page takeover, or letting users mysteriously figure it out... they promoted the change simply, cleanly and directly at the point of importance. If you had not yet clicked on a photo, Facebook revealed a message upon hover that clearly stated the new change. Once the photo was clicked, the new experience was obvious and the hover over disappeared. Well done.