No, it’s not safe to say that at all. First of all, not everybody who visits StackOverflow are professional developers. Many are newbies asking for help; they don’t count as full-fledged developers yet.
Second, it’s ridiculous to presume any kind of correlation between 55 million unique visitors and 19 million worldwide developers. Just in my circle of programming acquaintances, none have visited StackOverflow in the past several months (yes, I asked). This suggests to me that StackOverflow is not nearly as popular as you imply.
In fact, in my personal experience StackOverflow has not been particularly helpful. I often find the community to be arrogant and snotty. And I’m not the only one who feels this way.