The definition of URL states: "A contact that is not a phone, fax, pager or email address and is expressed as a URL. This is intended for various institutional or personal contacts including web sites, blogs, Skype, Twitter, Facebook, etc. Do not use for email addresses."  Therefore, the URL already encapsulates the concept of "messenger".  If additional clarification for the messenger URL is needed, an extension could be defined and used to convey necessary details.