Is there any way to make shared activities open up in Metric units rather than statute (when shared via a link). Is there something I can append to the URL to force it to be displayed in metric?
To set the display to metric go to Settings and set the Measurement Units to metric. This will only work if you have an account on GC. If you just follow a link without logging into GC then it will default to Statute measurements.
When you're logged in and you've clicked on the view in metric link, the address of the page at the top of the browser will change slightly to indicate that it is displying the units in metric. Copy that address and then send it as a link to Facebook etc. The parameters in the address will tell the Garmin connect page to display all units in metric for users that aren't logged in or don't have a account. The downside is that the address is really long instead of being the Garmin address then the identifying 8 digit number of the activity, so the full address might get split in transit.
For example below is a link to a run I did, it should display in metric, even if you aren't logged in. Works on a pc in Firefox and IE. Unfortunately it seems to revert to miles in Chrome nearly every time I test it, but sometimes it will display in metric.
