The real question, though, is that why would they want to do it?

A network that interacts in private is not what they want to make. The reason it took Twitter such a long time to introduce DM is that they wanted everything to be visible to everybody at first.

That way, you can befriend people you don’t know, and you get the power of the whole network for the inconvenience of a few trolls and some privacy. Twitter’s reasoning was, “if you want privacy, use email; Twitter is a network dammit”.