How does the scientific output of PhD’s who work in industry compare to that of PhD’s who stayed in academia (professors, mainly). I mean, in terms of number of publications, conferences, etc.
Disclaimer: this answer applies to computer science. I suspect things are dramatically different in the natural sciences.
It's hard to measure these things, and a bit unfair. If you work as a professor, publications are the base unit (along with funding) for measuring your performance as a researcher. You also are encouraged to grow research groups that in turn will produce more and more papers.
If you work in industry (even industrial research labs) (and I've done both), you are evaluated not just in terms of paper output, but in terms of internal metrics relating to how you help the company.
So the incentive structure in industry is different.
Bottom line, since I haven't really answered the question: I expect that the average number of publications/researcher will be slightly less in industry than in academia, but the average will get closer as the lab characteristics become more "university-like": think MSR vs (say) Walmart Labs.
I voted Suresh's answer up because its holistically very good and just wanted to add that in the field of scientometrics (a field I am somewhat familiar with and still getting familiar with), there has been some past work done on this general topic including academia-industry productivity and collaborations.
Reading some of these literature might be useful for you if you are looking at quantitative evaluations of such productivity and impact. Mind you though that quantitative evaluations do not always yield the complete or "true" story behind things. Regardless, this, this, this, this and this might be useful for you.
Note: This is only a small sub sample of such scientometric research and is not meant to be comprehensive or representative.
The ability of industry and other non-academic personnel to publish and present papers can often be restricted by intellectual property concerns, security restrictions, and other concerns. Therefore, even though the amount of high-quality scientific research being performed in non-academic settings may be comparable, the "measurable" amount—as seen in conferences, papers, and other "recorded" means—will often be somewhat lower than an academic group of comparable size.