Facebook Login Error

I have installed ‘Me Gusta’ theme on my new blog and also have set up a “Social Login” for WordPress so that users can register via Facebook/Twitter. But when someone registers via Facebook, after he/she redirects to the homepage, it doesn’t loads rather just below the login/profile button it shows this error :

Fatal error: Call to undefined method Facebook::picture() in /home/koundnya/public_html/GAGSONDUTY.COM/wp-content/themes/gagster/lib/php/main.php on line 132

What could be the possible reason for this ? I’m using facebook login. But even if also I don’t use any plugin for social connect, whenever I click on the login button at the top it shows the a warning at the bottom, i.e.
Warning: call_user_func_array() [function.call-user-func-array]: First argument is expected to be a valid callback, ‘facebook::login’ was given in /home/koundnya/public_html/GAGSONDUTY.COM/wp-includes/plugin.php on line 403

View the screenshot here : http://i1248.photobucket.com/albums/hh499/Abhisek_D/Warning.jpg

Btw my site is http://www.gagsonduty.com

Click on the login button and you’ll see the error!