I see you are using the alternate mobile layout. The layout that is delivered for mobile is in beta, it is not "mature" and it is not very well tested... It was intended to show that the functionality of "alternate" template
for mobile is possible...
Now, you have 3 possibilities:
1 Disable the "alternate layout for mobile" feature.
You can do this by removing the <mobile .../> element, inside the Site.config.
2 Fix this, by adding the following line in the mobile.js file.
$(".facebook a, .twitter a").attr("rel", "external");
(Between line 9 and 10, becoming line 10)
3 Change the mobile layout view (use another design; mobile js framework; etc...).
You can do this by changing the Mobile.cshtml located inside "/Views/Shared/" folder.
Hope it helps,