Facebook and Google are surely credible apps and other well known apps that wouldn't do anything out of the ordinary with your photos or what you give them access to if sure it's just so it can access them for you to utilize uploading photos or saving them to the camera roll.
Google and Facebook with both be collecting stats for advertising as jarlmaster has said. But I don't think you have to worry about them stealing or selling your photos. They have more profitable endeavors to keep busy with.
Yeah I knew they collected for advertising but I don't mind that. It makes it useful sometimes. But I mean as far as any malicious activity I don't see that happening with credible apps. App Store is supposed to be one of the safest place to get apps. Is what I heard anyhow.