Somebody and someone share all their definitions, and they are always interchangeable. When choosing between them, writers generally pick the one that sounds better with the surrounding sentence. This probably explains why someone is about five times as common as somebody on the web. Someone has fewer syllables, and writers presumably appreciate its brevity.
For example, this sentence sounds quick and breezy:
Others enjoy having someone in the car to argue with. [LA Times]
And this would be slightly clunkier:
Others enjoy having somebody in the car to argue with.
But someone doesn’t always sound better than somebody. For example, consider how much clunkier this sentence would sound if somebody and someone were switched:
There’s something that somebody doesn’t want someone to see at the Berlin Film Festival. [New Yorker]