When you paste a URL in the FB share, FB pulls out some information about the URL along with any available images.
I created a similar feature in ASP.NET few years ago and I got the project file while clearing out one of my disk partitions and thought of sharing it.
This project uses HTMLAgilitPack (http://htmlagilitypack.codeplex.com/) and XPATH for processing the HTML.
A working preview.
You can download the code here (http://sdrv.ms/MM5zA1) from SkyDrive.