The Internet Peering Ecosystem in the U.S. has changed significantly from 1987 to 2000, and even more significantly from 2000 to 2012. Much of this is documented in "The Evolution of the U.S. Peering Ecosystem" but you will also find some interesting differences between the U.S. Internet Peering Ecosystem and those found across Asia as described in "The Asia Pacific Peering Guidebook" where I document how peering in Asia is different from peering elsewhere in the world. Note that this work was done around 2006 and much has changed since then, but from this document you will learn some of the dimensions of differences as you build into other peering ecosystems.
If I was to generalize, I would say that we have seen churn in the U.S. Internet Peering Ecosystem as the result of these trends: