I think you have to think of the twitter experience as a progression where you start with people that you know/have heard of/can relate to, and over time you discover new interesting people in areas of your interest to connect with directly. So I feel like it makes sense to first suggest following a list of people you may recognize and want to hear more from, and from them get drawn into using the service more deeply. This applies to recognizable celebs / media / etc as well as friends. the company can do a lot of analysis to only suggest celebs, etc that do a good job of hooking users over time.

That said, having worked on some even earlier versions of this process (and with Ben too) it is a constant work in progress and set of changes needed as more and more people join the network. Today's users are later to twitter and probably don't want to just start and get drawn in following individuals but probably want content quickly that is personalized to them and would be open to learn twitter from there. But following accounts may itself be too much work. Or the challenge is just that mobile Signup is so common now that no one gets the experience and starting point from the web. In any case, the biggest challenge and opportunity with onboarding is just to constantly be changing learning and trying to improve it.