You post something, someone comments. You can reply to that comment or you can Like it. A Like says, "I saw what you said and thank you for commenting." You don't have to say anything else.
Even if you post a comment below the comment as a reply, if that person replies, normally you'd feel compelled to reply to that reply just to be polite. But where does it end? At some point you have to end the conversation. And that is when that Like button comes in handy.
I've noticed the absence of this Like feature in real life. Especially when emailing. So many times I receive an email reply that doesn't really merit a return email. I want to somehow let the person know I received the last email and acknowledge it, though, without filling his/her inbox with an email that just says "thank you" or "okay."
How cool would a "Like" be in that situation? Or in real life when someone comes to your cubicle at work and won't stop talking about nonsense? If you could just "Like" that person's last sentence and make them go away...that would be awesome.
Do you ever use the "Like" button on Facebook to gracefully end a conversation? Do you sometimes feel like you should send a short reply to an email to be polite, but you don't want to bother the person?