Lots of these answers no longer apply, so here's mine:
Use the Share Dialog described at the Facebook Dev Page.
Example:
https://www.facebook.com/dialog/share?
app_id=<your_app_id>
&display=popup
&href=https%3A%2F%2Fdevelopers.facebook.com%2Fdocs%2F
&redirect_uri=https%3A%2F%2Fdevelopers.facebook.com%2Ftools%2Fexplorer
But you have to put in your registered app_id, the href, and a redirect uri.