I'd say it's about 50/50. Some players need a huge push to interact with NPCs in a significant way (like Shalelu's lover). I don't push those too hard, because it's clearly not what they want. That's why that relationship is purely physical, because I also didn't want him to be left out of the RP scenes completely.
On the other hand, two players have made a huge effort to interact with their NPCs, hunting for presents for them, going out of their way to get them a share of the loot, putting themselves in danger unnecessarily on the NPCs' behalf. I reward that with a little more interaction. Both of those PCs are engaged, at this point.
Most of the NPC stuff happens in e-mail or on our Obsidian Portal forum, in part because I can't be bothered to slow the game down to do it in-game. Sometimes, though, I'll say, "the entire caravan stops today to celebrate the Ritual of Stardust. Everyone breaks out an instrument and takes turns playing and dancing, except Shalelu. How do you participate?" The Ritual of Stardust is great, because it's built around declarations of friendship and love, it happens every few months, and all the NPCs would participate (Ameiko's the only non-Desnan in the group, and if you can't get a follower of Shelyn in on a love-in, you're doing something wrong.) Come to think of it, that's the last big NPC thing I did, and the PCs have been driving the relationships ever since.