Someone that constantly posts what day it is on Facebook. This is usually done in a tiresome or annoyed manner, & often related to the number of remaining days of the workweek.
Examples:
Jen: I hate Mondays! Friday can't get here soon enough!
John: Jen, you're a real Facebook day dropper.
Suzy: What??? It's only THURSDAY?? Dang.
John: Ugh, again! Stop being a fb day dropper.
Brian: Tuesday done. Three more days until freedom...
John: Yes we get it Brian, you're a real Facebook day dropper.
Mary: It's Wednesday? Hurry up Friday!
John: Shut it Mary! Don't be a Facebook day dropper!