What you enter is different for each social service. Some require a URL, and some your user handle. To see which is which, look at the light grey example text in each field. If you already entered something, you can either delete it, or select it all, cut it, and paste it back if it was correct.

This image shows a typical setup. Note that Vimeo (4th from top) expects a URL but Twitter and Instagram need your user handle.

In the case of Twitter, Instagram and Snapchat, if you have correctly provided user handles and still cannot get your social buttons to work, try entering the user handles without the @ sign before them.