They don't like you

Get your résumé up-to-date and start looking for something else, because it's easier to get a job when you have a job. When it comes time to lay off people, it won't be your value or competence that will matter, it will be your popularity and you are lacking in that department. It would be better to leave on your terms and not on theirs. Standing on desk in the middle of the cube farm doing a crazy ranting rendition of "Take This Job and Shove It" while naked is a great way to go out.

Asshole coworkers. Damn.

Time to take a hard look in the mirror and figure out why you weren't invited. Maybe it's them but it could be your behavior isolating you from them.

Congrats on being "that guy". On a serious note, see why everyone is avoiding you. It could be confronting, but then you at least know. Perhaps your coworkers are indeed dicks, but if everyone avoids you, it makes you wonder...

Maybe make a little more effort in talking to your colleagues during lunch and such? Even then it's not the end of the world. I only went on staff nights out if I actually liked my colleagues.

well at least now you know to keep your work like and social life separate so you won't be disappointed next time.

A cubicle rager. Don't worry OP, it was probably shitty anyways.

Working on my college campus, people would organize parties all the time. I didn't get invited to any of them.

Eh I wouldn't worry about it. Office parties are office parties. You can have a much better time partying with your friends instead of backstabbing coworkers anyway.