That is, I meet people in a social setting, such as a house party, a the choir I sing in, people that visit my roommate, or even potentially just random encounters in a public place. Then, I establish contact via facebook for future social activity.
In many cases, these people are already friends with someone in my friend list. This means that tracking them down is very easy. (It is also easy for them to track me down, if they so desire). I look them up on facebook, friend them, and the next time I am interested in inviting them to something they are easy to reach.
That's what networking is, by your own definition: interacting with people in your fields of interest. And that's what facebook is good at. The list of dormant collections that is a typical friend list is still an excellent tool for establishing new contact, even if you don't technically 'meet' for the first time online.
The purpose of social activity is not research, or content sharing. The purpose of social activity is to connect with other human beings and share life, because this makes us happy.