Online slideshows etc

I'm real happy with the online versions of software I normally use on my own computer. The fact that there is no cost to use them is a plus. But more significant is the usefulness of being able to save your files on a shared server, without the responsibility of ownership of that server, is amazing. I saved about 50 photos in jpeg format on the Picasa site, while on holidays, as a backup to my own netbook. Those photos are still there, several weeks later. Being able to edit files & photos, and then save & email them, all without personal software or local email accounts, is a real bonus. The question in my mind: what's in it for these websites, if the users are not paying any money for the service?