I'm at 90% completion. I haven't done the Insane Stunt Jumps or Find all of the Pigeons, but I'm not sure why I don't have 95%.
I've beaten the game, stolen all of the cars, met and completed all of the Random Character's stuff, done the Vigilante (this is completed with the "Mean Streets" achievement, yes?), done the most wanted, and unlocked all of my friends abilities. I'm pretty sure that I've completed all of my friends activities.
Do you have to do all of the activities with all of your friends? The only one I might not have done is Boating but that doesn't seem like 5% would unlock after 1.