"Understanding" emotion is definitely a biological trait that computers are unlikely (maybe 2200) to ever be capable of replicating.
But lexical content is a very well understood field, and emotion recognition is a huge industry for advertising. Even if facebook doesn't know that "I like it when stan fingers my butthole" was just a joke between buddies... there is tone inferred from the words and grammar involved. Positive vs. Negative is relatively straightforward, and when associated with a topic... can lead to lots of "opinion" data.
But lexical content is a very well understood field, and emotion recognition is a huge industry for advertising. Even if facebook doesn't know that "I like it when stan fingers my butthole" was just a joke between buddies... there is tone inferred from the words and grammar involved. Positive vs. Negative is relatively straightforward, and when associated with a topic... can lead to lots of "opinion" data.