Homophily on the web - serendipity as antidote?

Another item from last Sunday's NYTimes magazine "Ideas" issue relevant to how our human minds organize themselves.....

Homophily refers to our inexorable tendency to link up with one another in ways that confirm rather than test our core beliefs. This trend is accentuated on "web sites like Facebook and MySpace, which tend to bring birds of a feather together. Meanwhile, chains of recommendations (“if you liked . . . ”) on sites like Amazon reinforce our original preferences even as they claim to expand our horizons." Social software designers who are behind sites such as there are questioning "how much they should encourage homophily and how much they want to mix it up."

What kind of software design might encourage “serendipity” to counter focusing on the familiar? " One information-technology specialist described a feature he would add to Facebook called “the Stretch,” which would help students “find a group of people a little different” from themselves. Someone else brought up the online book cataloger LibraryThing’s UnSuggester, which identifies the book least likely to share a library with the book you mention."