At the beginning it was simple, they were very few options. But now, there are quite a few photo sharing websites! They don’t propose the same features, and actually they don’t have the same objectives. Depending on what you are looking for, some will look much more aligned with your requirements than other. And you need to chose wisely as it takes time to upload your pictures, build your community and all the other social-stuff… Of course you can, and may work with several of them but again, you cannot just work with any, and every photo sharing website. So the decision is not so trivial!
As usual, I recommend having a look at Wikipedia for a quick and relevant overview of the main photo sharing websites.
Top ten reviews is providing an excellent and pretty extensive survey of several photo sharing websites. However, their approach is mainly if not exclusively “features oriented”: basic, photo, community, help/support.
Don’t forget things can change quite fast: a newbie can become very hype, some services will shut down too. I will do my best to update you and will provide my very personal and somewhat sarcastic comments about them! The main idea is just to give you a global view of these websites, the hot news and the trends. I am not an analyst, but I like to read their researches. It always helps me to make my mind.