Currently the externalLink template displays the image, title and description from a site in a div.

People will expect, and it would be preferable if it would rather display the oEmbed from popular sites such as twitter, youtube, vimeo where it is supported.

for information - see:
https://dev.twitter.com/web/embedded-tweets
http://oembed.com/

When not supported, it should fallback to the current output.