Pigeons.
But seriously, if they regularly have some form of broadband connection then I might e-mail them (especially if I need to attach stuff) or simply text them. You can tout the benefits of Skype, Lync, Facebook chat, Jabber (I had it before it took off), Google Hangouts (does anyone actually use that?!), Steam chat (!), but it's guaranteed that your (real) friends will have at least one e-mail account and that text messaging is more likely than not to be the last ditch in communications out in the countryside