Josh Clark, back in March:
Hey, please, under no circumstances should you pin social buttons
to the top or bottom of mobile screens. In an effort to try to
boost mobile use of share buttons, About.com experimented with
fixing them to screen bottom and separately to screen top, so that
the buttons were always visible when scrolling. While this did
modestly increase share-button usage, it also caused overall
session engagement to go down.
You read that right: adding a locked toolbar to the small-screen
experience shortened sessions and reduced page views. The very
small increase in share-button usage was far outweighed by reduced
site usage. (I can’t explain why this is the case, but I’ve seen
it elsewhere with locked toolbars, too. They chase small-screen
Read the whole article. First, Clark’s advice is based on actual results, not just opinion and hunches (like mine). Second, he doesn’t advise against ever showing custom sharing buttons — but he does say only to show them to visitors coming from social media referrals. And but even then, don’t put them in fixed position dickbars.
As for why dickbars actually decrease site usage, I think the answer is obvious: when people see user-hostile fixed position bars at the top and/or bottom of their display, especially on phones, they’re annoyed, and the easiest way to eliminate the annoyance is to close the fucking tab and move on to something that isn’t annoying.