Well... why not ask the king of search:
[4]
http://www.google.com/trends?q=blogosphere [5]
simply type your 1 or more keywords (separated by "," if more than 1) and voila:
[6]
Links:
[1] http://pulse.veltsos.com/category/it/blogging
[2] http://pulse.veltsos.com/category/it/google
[3] http://pulse.veltsos.com/category/it/search
[4] http://linux.philosweb.com/drupal5/files/image_2_35.png
[5] http://www.google.com/trends?q=blogosphere
[6] http://linux.philosweb.com/drupal5/files/image_4_10.png