Mobile devices are often drastically different from your average desktop device (where "desktop" can mean either desktop, laptop or maybe even tablet). The supermajority of social tools were essentially built in an age where few people even dreamed of the idea of using a mobile phone to view (and/or edit!) web sites, which rather directly translates to poor/unusable mobile UX.
Based on some quick testing with a relatively typical lower-end smartphone, the Nokia X, I collected a list of things we definitely should fix for mobile: http://social-tools.wmflabs.org/wiki/Social_tools_mobile_UX
There are probably a lot of other things we also need to do to make social tools usable on mobile; please feel free to open separate tickets for those.