According to Wikipedia - "As of December 2012, it has a total of 500 million registered users of whom 235 million are active on a monthly basis"
I do use it for another marketing tool for the business, however, I find that it is much clunkier than other social media vehicles. I have discovered with all of the other social media options, people don't choose to spend the time to use Google Plus.
I mostly use it to gain rankings with Google by linking out of Google Plus to my posts / websites. I do seem to get quite a bit of traffic that way. I have a hard time finding specific people as well, so I stick to Facebook for my personal networking and Google plus for one of my online marketing efforts.