I have the same question, specially if you offer your services for free to the users.. like facebook.
Performance appraisal at large corporations definitely suck. Reasons are many. But the point that managers or companies miss is the fact that performance cannot/should not just be appraised at the end of the year by collecting feedback. It has to be a constant process throughout the year. Most companies miss this trick. It is always easier to get better feedback when I delivered something by asking the team involved in it rather than waiting for next 6 months and go to them with a bunch of questions.Also, give feedback constantly. By the end of the year, it will be a breeze to determine the overall performance.
At Valve, apparently they're crowdsourced from all employees:
http://finance.yahoo.com/news/whos-boss-isnt-one-040100888.h...
You might want to look at 360 evaluations.