I noticed that facebook doesn't check my blog's RSS feed at a regular interval. I hadn't blogged in a while on my other blog, and facebook stopped checking it. It was only just now that it imported 4 posts at once. It can only mean that they're doing one of two things.
1) They're so overloaded, that they import peoples' blog RSS when they get around to it, and can spare some cycles.
2) They've implemented something like Ethernet's exponential back off when contention happens on the wire.
To me, #2 makes sense. RSS is known for its bandwidth hogging nature, since readers keep pinging the feed. I wrote about adaptive polling before, and it shouldn't be too hard. Based on a recent history of when someone posts on their blog, you can make pretty good predictions as to when they're going to blog next. Therefore, you can check the blog's RSS feed based on that, rather than wasting bandwidth. An easy way is to use a Bayesian classifier to do this.