They may be automatically posted by a robot. I'm on a number of business e-mail lists, and I keep getting ads for "push-button systems" that do everything for you, from generating copy to posting on forums and blogs. They promise to let you make a killing with less than an hour of work, but I can't see how they can do anything but create ill-will with their text that sounds almost right but tends to be closer to "colorless green ideas sleep furiously" than working ad copy. Computers may have come a long way since I was a kid, but C-3PO they're not.
All the ones I've gotten show no grasp of English.
Haven't seen any yet, but being friends-only is probably why. I just think of them as spam.
Buy more SPAM!