Omeka has encountered an error

Exception: String could not be parsed as XML
exception 'Exception' with message 'String could not be parsed as XML' in /var/www/html/omeka/plugins/SocialBookmarking/helpers/SocialBookmarkingFunctions.php:45
Stack trace:
#0 /var/www/html/omeka/plugins/SocialBookmarking/helpers/SocialBookmarkingFunctions.php(45): SimpleXMLElement->__construct('<html>??<head><...')
#1 /var/www/html/omeka/plugins/SocialBookmarking/helpers/SocialBookmarkingFunctions.php(55): social_bookmarking_get_services_xml()
#2 /var/www/html/omeka/plugins/SocialBookmarking/helpers/SocialBookmarkingFunctions.php(85): social_bookmarking_get_services()
#3 /var/www/html/omeka/plugins/SocialBookmarking/SocialBookmarkingPlugin.php(136): social_bookmarking_toolbar('http://archive....', 'Letter from Bar...', '2  page letter ...')
#4 [internal function]: SocialBookmarkingPlugin->hookPublicItemsShow(Array)
#5 /var/www/html/omeka/application/libraries/Omeka/Plugin/Broker.php(157): call_user_func(Array, Array)
#6 /var/www/html/omeka/application/libraries/globals.php(188): Omeka_Plugin_Broker->callHook('public_items_sh...', Array)
#7 /var/www/html/omeka/themes/bigpicture/items/show.php(86): fire_plugin_hook('public_items_sh...', Array)
#8 /var/www/html/omeka/application/libraries/Omeka/View.php(117): include('/var/www/html/o...')
#9 /var/www/html/omeka/application/libraries/Zend/View/Abstract.php(888): Omeka_View->_run('/var/www/html/o...')
#10 /var/www/html/omeka/application/libraries/Zend/Controller/Action/Helper/ViewRenderer.php(912): Zend_View_Abstract->render('items/show.php')
#11 /var/www/html/omeka/application/libraries/Zend/Controller/Action/Helper/ViewRenderer.php(933): Zend_Controller_Action_Helper_ViewRenderer->renderScript('items/show.php', NULL)
#12 /var/www/html/omeka/application/libraries/Zend/Controller/Action/Helper/ViewRenderer.php(972): Zend_Controller_Action_Helper_ViewRenderer->render()
#13 /var/www/html/omeka/application/libraries/Zend/Controller/Action/HelperBroker.php(277): Zend_Controller_Action_Helper_ViewRenderer->postDispatch()
#14 /var/www/html/omeka/application/libraries/Zend/Controller/Action.php(527): Zend_Controller_Action_HelperBroker->notifyPostDispatch()
#15 /var/www/html/omeka/application/libraries/Zend/Controller/Dispatcher/Standard.php(308): Zend_Controller_Action->dispatch('showAction')
#16 /var/www/html/omeka/application/libraries/Zend/Controller/Front.php(954): Zend_Controller_Dispatcher_Standard->dispatch(Object(Zend_Controller_Request_Http), Object(Zend_Controller_Response_Http))
#17 /var/www/html/omeka/application/libraries/Zend/Application/Bootstrap/Bootstrap.php(105): Zend_Controller_Front->dispatch()
#18 /var/www/html/omeka/application/libraries/Zend/Application.php(384): Zend_Application_Bootstrap_Bootstrap->run()
#19 /var/www/html/omeka/application/libraries/Omeka/Application.php(77): Zend_Application->run()
#20 /var/www/html/omeka/index.php(23): Omeka_Application->run()
#21 {main}