Generally, as a GM, I let players interact for as long as they seem to be, and when things slow down, I push the primary plot along. If players start interacting unexpectedly, I slow the plot down and let them build relationships. Every group is different. Some will spend a whole session (4 or 5 pages in PbF) bantering and getting characters engaged. Others won't post a thing if there isn't a GM-established conflict to deal with. In general, and in PbF in particular, I find it best to focus (as a GM) on propelling the plot forward. When things slow down, move them along. Use rich NPCs to show players that they can (if they're comfortable) elaborate on their character and interact outside the primary narrative. Beyond that, notice when things are slow and move the plot forward otherwise the game can fizzle out and fade away.
In this case, I'd say lets move the plot along.