I've seen stuff from a few people like Jeff Jarvis and Matt Cutts (Google Search quality body) implying the bubbling isn't too huge.
From my perspective, it has more akin to what the likes of Desmond Morris of the Naked Ape fame have been studying for years.
Also, one could similarly look at the likes of peoples' newspaper reading habits -
- people don't read the Grauniad because one wants or expects to see things counter to one's leftish viewpoint.
- people don't read the Daily Wail because one wants or expects to see things counter to one's rightish viewpoint.
Similarly in real life, one tends not to have too many friends drastically divergent from the views of ones-self (generally) - this tends to be how it is with people - we've always done it, the people doing the automated filters claim it isn't that heavy, and when we have curated content, we go for extremely bubbled filtering.
So in summary - Online Filter Bubbles?
Tim-meh
