Truthfully, I think it's hard to say from a one month number. There has been a sense that Facebook was the social network for the older demos and Twitter for the young. I do get the sense that the 40 plus crowd is finding Twitter. But my guess is this is most likely a blip, although 1.4 million is a lot of users, even when you consider Facebook's overall reach.
I've predicted this before; the Facebook pretenders(Google plus, etc.) don't pose a serious threat to Facebook. But someone, at some point, will come along with the next new thing. But that doesn't appear to have happened yet.