Generally I think hand signals are stupid. I see someone throw up a fist to signal no one is behind them when there's a sled right behind them all the time, and you should always ride as though there's an oncoming sled around the next turn anyway. When I'm alone or in a tight group I never use them.
That said I do throw up some fingers when I'm out front and I know my family is a couple turns back. As much as I hate hand the hand signals, I'd hate some asshole taking off and crashing into my mom because he thought I was riding alone even more.
As for people with mirrors not using the