A quick one while I have a moment. I want to write more about this later:
But a simple point I'd like to make is that we should view Facebook and Twitter as measurements of social interaction -- complete with measurement error and bias. Like Heisenberg's microscope, the measurement also affects how people behave. I know that much of my own Facebook feed has collapsed to marriage announcements, baby pictures, food pictures and vacations. People literally say they don't want to hear bad news in their feeds and tend to refrain from posting about negative events in their life (or alternatively, sharing every negative event).
Some of us only have Facebook friends we've met IRL as we used to say. Some friend anyone who requests.
As such, Facebook is obviously affecting the exact social interactions you wish to measure. Therefore it is likely only a good source of data for the interactions that are unaffected.
An analogy is a telescope with aberration. It's not good for panchromatic imaging, but at a single wavelength it can work just fine.