Bug 1024507: Measure use of Twitter and Facebook

<strong><em>IMPORTANT:</em></strong> After merging this, please remind me to update the client URL of my Facebook application. At the moment, to aid in testing, the application accepts calls from developer-local.allizom.org. Changing this takes no time, so if this merges while I'm on PTO feel free to just shoot me a text.
What are the advantages of using <code>.on</code>?