I've joined both and posted 4 images on the Flikr one ... but I've been on Facebook for some time now and still not found out what it's all about. It seems to me that it's for folks who have loads of time to sit and play on the 'puter ... which I do have but have other things to do instead Whatever ... I've joined them so I'd better get off out and get some new fotiz to go up on Flikr then
I think the majority of Facebook is not worth my time - however there are a few things (like Groups, Events and Galleries) that have at least some use.
Flickr on the other hand... I spend waaay too much time on.