Facebook: What is the purpose of the new "Like" button on Facebook mobile chat?

To kill the effect of a dangling conversation. There's often a sour feeling when you say something, know your buddy received and read it, expect an answer and then get nothing in return. A thumb up wraps up the conversation just right, and it's so hassle-free that your buddy might just tap on it. It's more of a feeling thing. I think, however, that Path's execution (little checkmark on green background) is more appropriate and less offensive. Facebook's thumb up looks huge on desktop browser (did they forget to shrink it?) and gives more of a "cool story bro" feeling. (photo from techcrunch)

Sometimes it's hard to get people to acknowledge something quickly. For example, I may send out a facebook message confirming the location of a meeting or dinner, but my recipients may not respond with a message. The like button makes it much easier to confirm or voice your approval for an idea.