Facebook has loudly and confidently entered an arena it has no prior experience in, and has set a clear path to expand its influence at its own pace. Facebook Home will provide a halo effect to current Android users that warms them up to a full-blown “Facebook phone” in the years to come. It gives Facebook the experience, confidence, credibility, momentum, and time to build a better and broader mobile experience than they would have been able to build otherwise. It’s as prudent as it is ambitious.
From everything I’ve heard from or about Zuckerberg, this seems like the kind of move he would make. Small step -> Collect data -> iterate -> more data -> big step.