I can see “Twitter” because they’re little people-chirps. And Facebook, since it was supposed to help you keep track of people you met.
But on the whole, perhaps it’s because other names are already overused, or somehow under copyright protection? My employer recently wanted to patent a technology under the name of a common noun, only to learn that the word was so general as to be unpatentable. That sort of thing. It has to be unique – which those names certainly are.
Our Fluther names are like that too, since we have to differentiate enough from other people to be recognizable. Why don’t we have 30 guys named “John,” since we probably really do have that many in real life? Instead we end up with names like “laureth” or “mollypop” because they’re unique, and can help tell us apart.