Pretty much all of the social networking widgets use remotely executed .js (javascript) which is called into the page on load.
Due to IE's security restrictions with Windows Vista and now Windows 7, they will not show in preview or render a placeholder image on the local machine (Xara's previws uses the IE engine in compatability mode) They must be uploaded to the host (published) first.
And yes, you can have more than one on a page.