I think they can be used interchangeably :)

Use "someone" (when you don't know who the person is, and there are too many people to choose from) Use "somebody" (is used with limited amounts of people are to choose from).

They're exactly the same and you can use them interchangeably. Some people say "someone" and "anyone" are a little more formal than "somebody" and "anybody" respectively, and other people say it's the other way around. It comes down to personal preference.

Both are singular pronouns, and use anyone when you are suggesting a person in a known group, as in, "Has anyone in this room been hunting lately?" And anybody when you are unaware of the group, as in "Has anybody ever gone thirty days without food and lived?" Similarly for (No one, nobody) and (someone, somebody)...