Michele,
We would recommend creating a plugin to do this.
Based on your description, it sounds as if you'd like to perform the action only on a social network login. In this case, you should create your own Social Profile plugin. The following link describes how to do this:
www.sourcecoast.com/jfbconnect/docs/gene...ing-a-profile-plugin - However you will not need to create all of the methods that link describes. You should really only have to override the "onLogin" method (found in /libraries/sourcecoast/plugins/socialprofile.php around line 128).
If you do end up wanting to perform the action on all logins (joomla and social networks), then we'd recommend you making your own user or authentication plugin (whichever one has the correct event for after login).
I think this should get you started, but please let us know if you run into any issues.
Thanks
-Melissa