Only 2 out of every 1000 mobile web users ever tap a custom share button. Don’t even bother including them in mobile sites except when users are coming from a social network.
I can’t think that I’ve ever clicked on a website’s social media share button, mobile or not, but I normally include them for my client’s sites as standard. To be honest, I’ll probably continue to do so as it’s something that client’s expect and I’ve got better things to do than debate their effectiveness. That said, it’s always good to have some statistics to refer back to.