You are floating the social media icons container without a width which means that the element will try and stretch to 100% if it can and when it does it no longer fits in that little space.
I've a feeling that IE may actually be the right one here as I would have assumed that the parent container should just keep stretching with the content and not wrap its internal content.
You will need to apply a width to the container to make iE behave as the others.
You may have to increase that width if you have margins applied.
Note: the XHTML 1.1 DOCTYPE you are using should only be used when Web pages are served as XML and not when served as text/html. There are some slightly different behaviours associated with this doctype also so unless you are serving the page as XML then use 1.0 or some other valid doctype instead.