fixed tooltip hover on unsocial send buttons

In response to https://trello.com/c/8MW3soSa/184-tooltips-for-send-this-in-share-bar-are-still-borkred

![screenshot 2014-06-18 16 38 43](https://cloud.githubusercontent.com/assets/1597375/3320935/2189f236-f731-11e3-9a05-a84f3d1ce573.png)
it's essentially left: 0 by default. to get it centered over a div, left has to be a negative margin.