To do that, you'd have to modify the JomSocial template to change the default link to go from JomSocial's invite page to a page you create with the JFBCInvite module.
To set up the JFBCInvite module on it's own page, you'd want to make a content article "Invite Your Friends". Then, you can put a little text in there explaining how or why they should invite their friends. Create a menu item (Article view) that displays this article. Finally, create an instance of the JFBCInvite module and set it to display on this menu item in a position that's in the main content area of your site.
Once that's setup, you'll need to determine how to change the link in JomSocial. I don't know how to do it off-hand, but can probably help you find it if you need. You may just like having it setup with it's own menu item (as described above) outside of JomSocial.
Hope this helps get you started!