I believe what Luption means by “linearity” is predictability, uniformity, and things that happen chronological time spans. Talking to a person is therefore linear, being predictable: Person A speaks, person B responds, and this flows in chronological order. There is no way to rewind time, no way go back and see anything that had previously been said. It is a predictable, uniform, chronological way of communicating.
Blogs, on the other hand, are not linear. You can move backwards in time, viewing posts from a time already passed. Categories, Archives, and even tags allow you to move through information in anyway you please, removing the linearity. Of course you can still view posts linearly if you wish too, but that is not the way an average person views the average blog.