Sorry,I don't really know the actual difference, but in my personal opinion, the "like" would be I guess if you "like" the general idea of a post, and the thank is maybe a higher-octane version of the "like"?

I know I hit the thanks button when someone has voiced something I agree with, or can identify with, or just thanking someone for replying to my post.

Just a thought, don't know if this is correct, but my interpretation, I guess.