You can use any service there is. Some have dedicated uploaders, some 3rd party uploaders and some you can just upload from their own uploaders. So, anyone you can think of... Picasa Web Albums, Zenfolio, Flickr, SmugMug, 500px, etc etc etc etc
If you are thinking of changing, can I seriously suggest that you create a trial library and import a couple of hundred images to start with. Explore the possibilities and note the differences. iPhoto is a dedicated photo manager. It's not about files. You import them and then forget about them. There's a conceptual difference between it and Picasa and some folks find that confusing.