Found this over at Alan Bevere's site:
It strikes me that only the two oldest Christian denominations, Catholic and Greek Orthodox, have anything like acceptable figures. And they both have clear standards and expectations of their members. We Methodists are not even retaining half. And almost every Christian denomination ranked lower than us has looser standards of faith and practice.
(I also wonder whether the source data also show how many Methodists, for example, stay active in the Christian faith, but in another denomination.)
Understand also that there is no functional, theological or philosophical difference between "nones" and "atheists." Combined, they retain the same percentage as Catholics.
Update: A lot more info about the study at Patheos, including these two observations:
The information comes from the Pew Forum’s U.S. Religious Landscape Survey — and there you can find a “Protestant” category. 53.9% of Americans say that they were affiliated with Protestantism as children, compared to 51.3% now — a drop of 2.5% in absolute value, or a decline of 5% proportionally. Most of the loss has come from Baptists and Methodists.
Also, while there are more people joining the atheist and agnostic ranks than leaving, it doesn’t bode well for atheists that 60-70 percent of those who are raised atheist renounce their atheism. If it were so compelling a solution over the long term, one would think the retention rate would be higher.