[tehjoker]

This seems right, but I think it's also about authorial control of format and the expectation of readers that they are going to possibly read longer texts. Twitter is very up front about formats and character limits. Facebook is less dramatic but still very self contained. The placement alongside many other items that scream for attention, the possibility that facebook won't present it to your audience, presentation, and interactions mean investing in forms of writing outside the constraints of the medium is just not very worthwhile.

There are places in Facebook where longer form writing is possible, but the system still doesn't really encourage it.