This is updated randomly, and can be as long as 11 months between updates. PageRank also happens on a Log Scale, though not a Log base 2. PR mostly uses a base 8-9. This is not obvious from the toolbar.
There is a low correlation between the Toolbar PageRank and real PageRank.
A number between 0 and 1. This is updated multiple times a day.
Pagerank is good as a rough indicator of a ‘raw’ link popularity. It can be useful to compare with other metrics, like mozRank. History of PageRank can be useful for insight into how the changes you have made have affected your PageRank.
Do not use PageRank for…
Understanding why things rank well in SERPs or not.
Valuing link prospects – check other metrics as well.
Reporting as a key metric – it will not send you more business, give you Twitter followers, it will not benefit you in anyway.
Note: PageRank is for PAGES only – it does not measure the importance of domains.