Cause of Google page rank Drop

In the above video Matt Cutts describes how a website or webpage lose its Google page rank. Suppose somebody use the Google toolbar know and monitor the Page rank of a website or some web pages. And he has known from Internet that the Google tool bar gives old and quite unreliable data about Google page rank of a website or a web page. If there is any reliable source for knowing real time page rank information about a website or web pages he manages or monitor? And how he or any SEO Expert can identify or know the causes of a page rank drop or its causative factors

Watch a veido saying the relevance of Back links from Social networking websites like Face book and Twitter