Use embed.ly by default if turned on.

WordPress has some issues with "for" parameter encoded in final URL fo some reason. 
So the request looks like this: http://public-api.wordpress.com/oembed/1.0/?format=json&url=http%3A%2F%2Frodp.wordpress.com%2F2013%2F02%2F27%2Fon-scorpions-and-frogs%2F&amp;for=wpcom-auto-discovery
But the last "for" parameter should be a part of API URL, not as part of blog post URL, otherwise WP returns 501. Embed.ly handles this flawlessly at least..