I can't pass comment on ImageShack as I don't use it... but Flickr is great! 🙂
I've been a member for a while now, also got a few people from my family on to it as well. There are some GREAT photographers on Flickr - it's a great way to learn new techniques, get inspired, share your own photos or ideas, and receive feedback.
The Explore feature picks out 500 photos a day that are considered "interesting". Flicking through these can be quite inspiring in itself, and, if you are fortunate, your own photos might be picked to appear in Explore too.
One word of caution: there are a lot of great, well meaning folks on Flickr who are in it for the "social" side - rather than serious photography. These will happily make friendly comments galore - but never give constructive criticism, or maybe aren't even particularly clued up on photography. It depends what you want out of it really, and how seriously you take photography. Pick your contacts carefully, and you'll find it really helpful and inspiring.