For Google Reader's case, it tends to display more to the top of its home page of the kind of news you click the most (not really _filter_ per se, just make them harder to find). For Facebook, it hides your friend's updates if you don't click on them after a while. Lookup Eli Pariser's TED Talk somewhere from the web (the talk is actually _removed_ from TED's website, which shows the power of Google's & Facebook's corporate muscles in doing censorship).