While it's not my primary means of communications, my Facebook account gets near 0 spam (both literal spam and "mailing lists which are fairly boring but every now and again I should check") which means that while I made everything else in my smartphone just give a little "pop" sound, Facebook messages (and SMSs) actually make a "pring-pring" (more noticeable) sound -- if my Facebook mailbox has something in it, there's a high likelihood it's a real person, needing something real from me (and there's a high likelihood that I want to give them that, since I actually know them...)

My point: It's actually rational to assume FB messages are more valuable, and given people who do that, it's equally rational to send them messages on FB :)