Application.OpenURL not working as a Facebook Canvas Game?

I'm testing a web build of a game as a Facebook Canvas app, but once the game is on Facebook, all external links stop working. If I click a link, the game stops showing but the browser remains on the Facebook app page. The links work fine when playing in the Unity editor or as a published web game on my website, it's only once it's on Facebook that it stops working.

Is there an alternative to OpenURL, or something else I can try to get this working?

EDIT: I've got it working using - Application.ExternalEval("window.open('http://www.google.com','_blank')"); but this opens in a new window/tab which is usually blocked on a browser. It does however at least work, you just get the popup prompt in the browser.