When sharing a vialogue link on Facebook, no thumbnail or video meta data (title, duration, the fact that it is a video) appears. This is unfortunate because many people do not know that a vialogue link is a video and without the thumbnail, it is easy to overlook the post. I think this is a lost opportunity for those trying to share vialogues and for the platform as a whole. You can read about Facebook meta tags here:

http://developers.facebook.com/docs/opengraphprotocol/

There may be other meta tags for other sharing sites, but I am not familiar with them.