When you like someones status comment, picture comment or wall post on you're facebook.
You don't do it just because you like the post or comment but because you envy it so much you can't think of anything better to comment.
for example if their joke is too funny and you can't top it, instead of trying a comment back you just like their comment, to save perhaps embaressing yourself or wrecking their comment or post.
Other examples are when theirs no need for a comment and you're just showing you're envy of their facebook comment or post. this can be on your facebook or someone elses, see example 2.
the jealousy-triggered condition from which other Facebook friends suffer when comparing their own paltry number of "Likes" on a status update with someone else's, posted by a person who consistently reaps "Likes" by the dozens.
Jimmie: Holy crap! Did you see how many "Likes" Jill got with her status update about her amazing landscaping skills?
David: No! I'm still reeling over how many she got when she posted a picture of her sneakers!
Adam: Guys, guys. Just listen to us here. What have we become? We have a full blown case of Like Envy.