In the past 24 hours, 2 of my facebook friends have posted a spam comment on my wall:
“Found some super potent legal bud at http://www.b_bzop.co_ !!”
So it would appear that either their accounts have been hacked, or they have some spy ware on their machine that is hijacking their sessions and posting comments while they browse around the site.
If you think your facebook account might have been hacked, then you should change your password to a more secure one, and run a virus checker on your machine!