Social bookmarking---comparison and purposes

I've searched through the archives and I can't find one place that really summarizing the thinking of the group and the group's experience with social bookmarking sites.

I'm trying to discern whether I should use delicious or pageflakes or diigo. I get that delicious doesn't allow you to share anything but the annotation and the URL. It seems as if pageflakes allows you to do that and adds more options (podcasts, RSS feeds). Diigo almost has another purpose...to save teh URL along with pieces of the page.

What would you chose to use and at what age group? If I'm repeating a question that has already been addressed, if you could tell me where to look, I will go there. But I couldn't find anything that summarized all three of these in one place.

Thanks for your help.