It seems there is less activity because people cannot find questions they like to answer. The whole topic thing is a little bit confusing right now, but as we get used to it, it will sort itself out. But I'm sure just as many people are on as there was before the switch.
It's not as much fun now to answer questions. Also, the site is more complicated. I don't even have it figured out yet. I've not even been to my own page yet or does it even make any differences or who your friends anymore. I also wonder how so many people accumulated so many exper points... like way over 100,000. It'd take over 3-5 years at 100 points a day to accumulate that many. That'd be answering 30 questions every single day plus asking questions plus getting some best answers. But maybe this is all some people have to do, plus this is just a fun site so question answers shouldn't be taking many of the answers they get seriously. Anway, that's my take on this.
Yes, to me there does seem to be slightly less activity, especially from the "regular" users. Since the new site launched, I rarely see users that posted fairly often on the old site. There seems to be a lot of "new" users on since the upgrade.