Same place. Look at the number of student pilots over time, at the one year jump in 2010. Here's the explanatory footnote:
1/ In July 2010, the FAA issued a rule that increased the duration of validity for student pilot certificates for pilots under the age of 40 from 36 to 60 months.
This resulted in the increase in active student pilots to 119,119 from 72,280 at the end of 2009.
Also, go to table 22, Student Certificates by month. Here's the annual totals:
YEAR 2016* 2015 2014 2013 2012 2011 2010 2009 2008 2007
Total 36,145 47,381 47,407 49,566 54,370 55,298 54,064 54,876 61,194 66,953
Remember, the way student certs changed in 2016, so don't get too concerned about the big dropoff.
The other thing I look at to get some idea of general aviation activity is avgas usage. The best figure I can find it the US EIA's Product Supplied one:
https://www.eia.gov/dnav/pet/hist/LeafHandler.ashx?n=pet&s=mgaupus1&f=a
There's been a steady decline over the last three decades. Some of this is due to the greatly reduced number of working planes that are piston powered, but I think that's about done. If you look at FlightAware during the business day, very few of the piston aircraft that I'd expect would be professionally flown are in the air at any given time. Other than Cape Air's fleet of 402s, I don't see many of the bigger piston twins there. There's a reasonable number of aircraft that I would assume are being owner flown for business, mostly SR22s, but also Barons and Bonanzas, and a decent number of Skyhawks that I assume are mostly doing training. Anyway, that figure is declining at around three percent per year, which is slightly larger than the decline in the pilot population.