I never post grades on FB. Sorry, but I think that's obnoxious. You wouldn't put your performance review from work on there, right? Grades are something private - if a kid gets high honors and it's in the newspaper or something then I guess that's different but I cringe when people post about their kids' grades.
In general, I don't post results of tryouts and such (or even talk about them online or in real life) because I know that there are other kids my kids and I know who didn't make the team, or the band, or the play. Now once they're on a team or in a play or something and I'm actually at a game or performance I'll sometimes post about the results or share a photo or short video clip. Same thing with things such as inductions into the National Honor Society - I shard a photo of our child and her friends dressed up for the event and congratulated all the students who had earned that distinction. Less about our kid and more about sharing a positive community event.
My SD won an essay contest last year at the state level that earned her a trip to Washington, D.C. to complete on a national level and of course I shared that, but it was also covered by our local paper, her school, etc. as she was the first contestant from our region to win the state contest. Her robotics team also earned a spot at a national competition a few years ago and I shared that too, but that was also covered by the papers and the school because it was a big deal and the first time they made it that far.