From ESPN's anonymous survey of Major League Baseball:

In your best estimate, what percentage of MLB players are still taking PEDs? 

AL outfielder: "Twenty percent. Higher for sure than the number of ones that got caught. Those guys only got caught last year because an employee leaked all the records, otherwise they would have gotten away with it. Look at Ryan Braun. He was on one of the most complex doping regimens there is, so just take that and you know there have to be more." 

AL slugger: "I think the number is probably about 10 percent, but that's not counting Adderall as a PED. If you included guys who are using Adderall, wow, that number would be through the roof."

Here's the link to the whole survey.

On the bright side, there's this fantastic video.  Full credit to you, Mike Polk.