first of all, thanks to everyone helping around here, you guys are great.
When I started using wordpress.org I chose Shareaholic to share contents in social networks. It shared correctly but didn't attach any thumbnail to facebook shares, so it looked quite ugly.
I started reading then about "Open graph", where the problem seemed to be. For that reason I installed Yoast and that solved my problem. Now, after updating to WP3.5, when I share on facebook a post with a picture attached, works perfect. But if it has embedded video, it attaches no thumbnail. So in those posts I have to add "featured image" with a screenshot of the video, so I can get a thumbnail of that post.
Any solution please??