Facebook is a good vehicle for reconstructing your own historical network. As we go through time we develop a historical network through family, friends, geography and institutions. Facebook nicely allows you to reconstruct that network and participate with people you’ve lost contact with. Twitter, especially combined with the search capacities helps you to develop a network around common interests with people for whom there is no short historical route with which to connect. If you tweet enough on a subject and someone is trolling for things in the stream for things you’re tweeting it’s easy to follow and to then continue to construct your network. That network will of course grow as people begin to find each other and read their tweets.
The networking economy of Twitter is more fluid and pragmatic. There is a social cost to “de-friending” someone on Facebook because you likely have a historical connection this this person and that network. Twitter “followers” are much more about the signal to noise ratio of a person’s tweets. This means that you’re always refining, adjusting and likely improving your personal network for what you are trying to get out of it. Even the naming “followers” rather than “friends” makes it easier to stop following with less risk of stigma or embarrassment. The relationship likely hasn’t been “historified” in the way Facebook relationships are through pictures and other cultural artifacts.