[mr_b]

I think both cater to a different set of people/photos. I upload only those photos to Facebook which have a 'social' element - Birthdays, weekend trips with friends etc. But Flickr is more for photos which I click as a hobby and as a photo backup service.

[davidedicillo]

Also on flickr there are a lot of pros using it to maintain their portfolios, but definitely I think Facebook photos are one of the reasons of its success.