Most large content providers use some sort of egress traffic engineering on edge web proxy/caching servers to optimize the end-user experience (avoid congested transit autonomous systems) and link utilization on egress links.

I was planning to write a blog post about the tricks they use for ages, and never found time to do it… but if you don’t mind watching a video, the Source Routing on the Edge presentation Oliver Herms had at iNOG::14v does a pretty good job explaining the concepts and a particular implementation.