I'm using something like:
http://www.facebook.com/sharer/sharer.php?u=http://www.google.com
to share Facebook Pages. This outputs as follows.
Is there a parameter I can add to sharer.php in order to default the "who can see it" dropdown (on the top right) to Public?
The sharer.php is being deprecated and you should use the like social plugin See https://developers.facebook.com/docs/reference/plugins/like/
EDIT
Official word from Facebook on the sharer.php being deprecated and not supported: http://developers.facebook.com/docs/share/
The Share button has been deprecated in favor of the Like button, and will no longer be supported.
Please use the Like button whenever possible to drive maximum traffic to your apps.