Error is telling that for some reason you are still using the widget that belongs to "OCS GUESTS" plugin while, most likely, plugin is inactive or has been uninstalled.

I am not sure what is the name of the plugin exactly. Most likely it is this one: http://www.oxwall.org/store/item/166

What did you do before you started getting this error? Did you try performing any updates?
Is the "Who viewed me" plugin active on your site?

If i am correct here the issue is with facebook connect, disable the plugin and contact the developer...   team please verify thanks 

-1 for me lol darn 

The issue is related to some OCSGUEST plugin, is it active on your website or you have disabled it, if so, please reactivate and try again. If the issue persists, you will need to contact OCSGUEST plugin developer to resolve the issue. 

oh so there is a plugin named that ok got ya i didnt catch the name of the zip file on that page.  thanks ross and +1  Ox tips nice job... :)

It was not "who viewed me " it was cos_topusers tnx for help

thanks for sharing the results... glad you got it fixed. 

Great. Thanks for letting us know. I'm going to mark this thread as Solved then. 

In my case it is with the "Who Viewed Me" plugin. When I updated to 1.8.1, and went to re-activate it, it threw a debug error. Apparently it is trying to install a duplicate key in the database. "ocsguests_menu_item"
There is one there that has a type of "Mobile_Top". The others that have this type have "mobile" in the menu item key. Looks like this one is trying to use the key of "ocsguest_menu_item" for both regular menu, and mobile. I sent the info to the dev, so we'll see what happens.

Tammy noticed the deactivate.php didn't have a line to remove the menu entry from the db table. After adding her fix for this the debug error went away.

Added this in the deactivate.php
OW::getNavigation()->deleteMenuItem('ocsguests', 'ocsguests_menu_item');