If you delete these rows, your site will just get slower, because WordPress uses transient options for caching. These options will be deleted automatically when they time-out. See here for more information.
No, it means if you delete these options manually, your site will get slower, because the plugin has to communicate with Facebook again to get data.
The transient options temporarily store data retrieved from Facebook.
A large number of options should not slow down your site, because WordPress does cache most options, unless your hosting server has not much memory available, but that is something only your hosting provider can fix.