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They've evolved into a better way of expressing certain emotions and such.

For example I've seen things like "I'm sorry you're leaving! #dumb" which helps convey that the person is laughingly sad that the other coworker is leaving.

Stuff like that.

[roryreiff]

I think this is the way that most people come across their usage and start using them themselves. It's almost a "under the breath" type of comment. As in "Eating dinner #ImFat" or "About to watch Halloween movies #soScared"