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.
Not to be confused with a Pitty Like
Mark's comment: Awh man i I love that movie
You're comment: You know how i know you're gay?
Mark's comment: Because you're gay and can tell who else is gay?
*You like this comment*
Dude1: Man mark tottaly got me in my status about that new Judd Apatow movie, I had to give him an Envy Like.
Dude2: Yeah I seen that, it was a clear envy like, that could be embaressing if you guys werent good friends..
Craig's status: Just had amazing pizza, weed and sex with my girlfriend multiple times on each front.
*You like this status*
Guy 1: Man did you see criags latest facebook post? sounds like he's having a great day, i Had to give him an envy like.
Guy 2: Fuck, I can't wait to get home and envy like that shit.