Facebook Timeline, the new profile design launching soon on Facebook, makes broad use of the Bing Maps API. Similarly, the Facebook Places feature also incorporated Bing, but now both maps use a different, more simplified style.
The new style most notably does away with the purple freeways the clashed with the social network's muted blue color scheme. Smaller roads are also given a subtle shading at outer zoom levels, which makes for a less messy map.
Similar alterations are easy via Google's styled maps, but Microsoft has no documented feature to change its map tiles. It's likely we're looking at a special set that Microsoft, which owns a stake in Facebook, created for the social network.
Bing tiles normally contain a stl=h parameter. On Facebook, that is changed to stl=fb. Though Bing obviously has the ability to change styles, it is likely a somewhat manual process. However, it would be great to see this feature come to its mapping API to make it more competitive with the Google Maps API.