some would construe this as a gesture of more faith -- not less. it's generally wiser to put the best people on your biggest opportunities, not your biggest products. sometimes, they are one and the same, but often they diverge.
the roadmap and strategy for messenger are defined. marcus oversaw a bunch of experiments, and it seems like the results are in and the experiments over. what you need now is someone who can execute the messenger strategy.
bitcoin, however, remains undefined. it could be a blue ocean of opportunity and reshape the world; it could also be a wasteland and memorialized as digital tulips. the last thing zuckerberg wants is fb degenerating into ballmer-era microsoft or fiorina-era hp, ignoring tomorrow's dollars for today's dimes.
someone like marcus could retire and do whatever, so if he's staying, it's likely because he both likes it and sees the opportunity. what's more impressive is zuckerberg convincing marcus to stick around.