I've had a browse around the Visual Studio 2013 online capabilities but can't see any wiki as such. By wiki I mean somewhere to capture enduring product related info e.g. info on how to support product xyz, how to setup a new dev workstation, how to workaround that annoying "msshrtmi.dll" Azure bug that continually crops up. Stuff like that.
Currently using Google sites wiki which is awful!
Unfortunately the Visual Studio Online does not support wiki for project now. I miss this feature too. You can vote for the feature on uservoice to accelerate its development.
Update: Issue is completed on 9 January 2015. Thanks to developers and to all who voted. For more information, see http://blogs.msdn.com/b/bharry/archive/2014/12/18/improved-welcome-wiki-experience.aspx