[mkuhn]

The filtering is basically done by the people you follow. They add what they find interesting and that is a pre-selection.

[chmars]

Most users seems to add all links from their tweets. Maybe that will change!

[mkuhn]

Not in my experience. But as I have mentioned in my own top-level comment, I have been fairly diligent about unfollowing the people that produced noise in the stream.