People using facebook has externalities, those externalities affect me, which is why it matters to me. Most obviously, it's a proprietary communication service,

People using facebook has externalities, those externalities affect me, which is why it matters to me. Most obviously, it's a proprietary communication service, and communication services tend to exhibit a network effect[1]. [1] http://en.wikipedia.org/wiki/Network_effect http://en.wikipedia.org/wiki/Network_effect