Nathan Torkington - Homophily in Social Software:
"If you prematurely narrow in, you'll end up only showing them stories about melting Antarctic ice shelves without connecting to the rest of environmental, travel, or scientific stories that they're really interested in. The best way to make those connections is to mix it up.
Doing this creates serendipity: pleasantly surprising the user. For example, don't show just the top 10 most similar items in your recommendations list, but show the eight most similar and two from the mid-range. Or call the "less relevant but also likely to be interesting" results out like you're advertising them: put a heading like "Take a walk on the wild side" or "Break out" on top and act like it's a feature you're offering, not a bug you're fixing.
Breaking out of the tight circle of self-similar recommendations is a feature."