Used when a family member (usually a parent, grandparent, aunt, uncle, or some other older generation family member) posts something online, usually on Facebook or other social network, that pertains to you in an embarrassing or disapproving manner.
Posts are almost always linked to you or on a thread in response to something you posted. Famtroll posts usually consist of an embarrassing picture of you as a baby and referring to you as "My little boy/girl", disapproving responses to your posts, or telling how much they "Wuv you shooo much my little (childhood pat-name)!" on your profile page.
This usually stems from older generations' inability to understand how the internet works on a social and human interaction level, as well as your stupidity to have your mother/father/grandparent as your friend on Facebook.
Steve's Mom (via facebook): Uhhh... we have to talk about this. You shouldnt be posting stuff like this on the internet. And what did I say about posting swear words! Luv U, xoxoxoxoxox -Mom
Steve (via Facebook): OMG Mom, WTF?! Are you kidding me right now?!
Hank, Steve's friend (via Facebook): HAHAHAHAHA!!!! FAMTROLL'D!