I use Flickr because it brings a lot more exposure than Picasa Web.

If you only aim at sharing pictures with family and friends (full albums), then go for Picasa. It's great for say, people who shoot a whole lot of pictures, as the format is adapter for it (for example, one album per night out).

Flickr is more for people who regularly take pictures, say an average of 5 a day. The photostream is adapted for such a scenario.

Being able to share specific pictures in pools around specific categories in Flickr also enables, as I said to begin with, greater "exposure" if you think your pictures are worth sharing (that is, pictures with specific qualities to them. Your buddies drinking or jumping in the pool rarely has anything worth sharing except for the people who were at that specific evening).