Staffa - 8:22 pm on Jan 9, 2012 (gmt 0)
Thanks keyplyr, I understood that and it's exactly what is happening for the moment.
Because of the competition, some participants posted a link on FB to my web site. I guess they probably are urging their friends to go and vote for their entry and the friends come with a FB referral URL.
facebookexternalhit precedes these friends and arrives at the same time but always before the visitor. It comes from the three ranges that you posted.
But, as yet, no FB visit without UA to grab one or more images.