Classic Protestantism and Anglicanism have dropped like a stone and now only make up 26% of American Christians. Catholics and Orthodox grew dramatically and together now comprise 38% of all Christians in North America. The new post-Protestant groups have come out of nowhere (Independents and Marginals) and now make up 36% of Christians.
Independents and Marginals still have Reformation DNA because they emerged in reaction to classically Protestant Christianity but most do not see themselves as "Protestant" in the way the term was used in 1910. They are the more or less estranged children and grandchildren of historic Protestantism who no longer feel bound by historic Protestant creeds or consensus. We continue to see them as "other" because we are highly sensitive to their still existing Reformed assumptions, but their recent past and their future trajectory is away from historic Protestantism. Because they are passionately evangelizing and masters of the media - old and new - they are growing faster than all other Christian groups.
Now Brandon makes a point in the comments that these new protestants are not fundamentally different from the old protestants. They share the same underlying assumptions. They have just taken them a step further. He is right. But the extra step or several steps are important to note. The more liturgical and more creed-centered churches of 100 years ago were stronger because of that. The rate of change over the preceding centuries was pretty slow. Now they have lost that and the rate of change has accelerated dramatically. When you talk about protestantism crashing and burning in the next 100 years people reply that it has been around for centuries and will continue for centuries more. But it hasn't been around for centuries. Not like it is now. It is not a solid rock. It is a house of sand that it crumbling over time.
Solid creeds and liturgy that helps the church pray her creeds is so important. It allows us to internalize what we believe. It helps us to pass it on to our children without changing it. It allows us to grow together and towards God slowly and organically rather than demanding a sudden conversion of life.
I was raised in a reformed church that was very liturgical and very creed-centered. It would still show as such on Sherry's study but it is much much less so than it was. Some congregations are independent in all but name. Those are the fastest growing ones. I think the same can be said about the Lutheran, Methodist, Presbyterianism, Congregationalism, and Baptist churches. Even those have become less creedal and less liturgical. Sure some traditional pockets remain but the excitement is all around the funky new churches that look and feel like independent churches even if they are technically mainline. So I think the stats don't tell the whole story. If that dynamic is accounted for the shift in protestantism would be more dramatic yet.