According to Fox, MLB.com charted 11 of Santana's outings last year, including his relief effort in the All-Star Game. Of 1,033 pitches, Santana threw 61 percent fastballs, 27 percent changeups and 12 percent sliders, which comes close to Bill James' full-season tally (58-29-11). Not surprisingly, Santana used the changeup far more against righty hitters (he threw it 33 percent of the time) than lefties (7 percent).
Just as Morris suggested, Santana does love using the pitch for strikeouts. Of 269 situations that Fox charted in which Santana had a hitter at 0-and-2, 1-and-2 or 2-and-2, he threw the changeup 123 times. Of the 86 strikeouts Fox witnessed, the changeup produced 53 of them.
Interestingly, of the 11 home runs Santana surrendered on non-full, two-strike counts on Fox's watch, just two came on the changeup, with eight from fastballs and one off a slider.
It turns out that Mike Fast did a nice analysis of Santana back in January and as you would imagine found essentially the same thing albeit in much more detail. From a start by start basis the mix of Santana pitches in 10 of his starts and his All-Star appearance last season can be seen below.
From this it is not apparent that he increasingly used his changeup as the season wore on and in fact it shows a trend where he used his fastball a bit more as the season progressed.