MGMA data is available by subscription only (and it's expensive.)
This should be useful, however: http://www.allied-physicians.com/salary_surveys/physician-salaries.htm
Why on this survey does the FP w/o OB drop >3yr?
Nobody really knows. I have a theory that it has something to do with high starting salaries in managed care, but that's just a guess.
I've never met anyone who was making less money after three years or more in practice than they did in their first year, but then again...I don't live in California.