The real issue, it seems to me, is not whether Facebook makes us lonely, but whether Facebook is reconfiguring our notions of loneliness, sociability, and relationships. These are after all not exactly static concepts. Here is where I think Marche raises some substantial concerns that are unfortunately lost when the debate goes down the path of determining causality.
What Facebook offers is the dream of managing the social and curating the self, and we seem to obsessively take to the task. The asynchronicity of Facebook is rather safe, after all, when compared to the messy and risky dynamics of face-to-face interactions and we naturally gravitate toward this sort of safety. I suspect this is in part also why we would sometimes rather text than call and, if we do call, why we hope to get sent to voicemail. It seems reasonable to ask whether we will be tempted to take the efficiency and smoothness of our social media interactions as the norm for all forms of social interaction.