there may be something wrong with the tool you used to check popularity. there can be changes in number of backlinks. but it wont reduce drastically as you said
Regards your site's PR so did you change recently in title, keywords or any content of your index page. and for backlinks so if you've used some free resources so need to check manually for the reasons of droped backlinks. Thanks John
Which tools you used to check the backlinks? Use yahoo site explorer, it might show you all backlinks. I am not sure but Google might be giving importance of aged links only.