Pfui - 11:54 pm on Feb 3, 2012 (gmt 0)
1.) "Darwin" is also used on/by multiple Mac platforms to retrieve favicons. E.g.:
Safari/6534.51.22 CFNetwork/454.12.4 Darwin/10.8.0 (i386) (iMac10%2C1)
Blocking it just adds to log bloat with the additional steps.
2.) Lucy, I don't know why the Fb app sent 400s, just that it did.