Be mean

Don't be mean.  There's basically no justification for it.  

Facebook Events

If you're invited to a facebook event, and cannot go, you don't need to brag about what you'll be doing instead.  I've seen so many people do this, and afterwards, I hate them secretly forever.  They say something like:

"Sorry I can't make your birthday party!  I'm going to Puerto Rico and then Argentina for a month or else I'd come.  Have fun!"

What the hell?  No one wants to hear you brag about what you're doing and why your Friday night will be better than his or hers.  And you wouldn't tell a room full of strangers your plans for the weekend.  Just write something like

"Sorry I can't make! I'll be out of town but happy birthday!"