If there's one thing to remember about online photo sharing and storage, it's this: The services and options change as quickly as the weather. So, too, do their terms of service, which can be a sticky, confusing mess that results in other people using your pictures for their own profit.
Understand that more than a few social media sites (such as TwitPic and Instagram) make picture sharing very simple and fun. But use these services with this caveat -- you may be giving that site permission to use or distribute your pictures in any way they see fit.
Protect yourself by reading the terms of service or by searching for TOS complaints regarding specific sites. One upside is that these sorts of TOSes are more common with social-media sites than storage-centric services.
Finally, keep in mind that there isn't a one-size-fits-all photo storage and sharing system. Your needs may even change depending on the project. To wit: Facebook is great for random snapshots, but Flickr is sleeker and slicker for a burgeoning album of vacation photos.
No matter which road you take to the cloud, you should be excited for the future of digital photo fun. As these services expand and companies tweak their business models, your options will continue to increase exponentially.