P(A) be the probability that a given internet user has the Alexa toolbar installed, and let
P(S) be the probability that a given internet user is part of Stack Overflow's core audience.
Here in Germany a poll is usually considered statistically valid if about 1,000 out of the 80 Million people have been asked. So let's assume 0.001 % (or one in 100,000) of people have to be asked for statistic data to actually have a meaning.
Alexa states that
it is difficult to accurately determine the ranking of sites with fewer than 1,000 monthly visitors
In other words (we're of course simplifying a lot), Alexa considers their statistics for a particular site to be reasonably accurate if 1,000 people per month have been "asked". Combining this with the above number means that the number of internet users Alexa knows about is
N = 1,000 * 100,000 = 100,000,000
According to the ITU, 22% of all people were internet users in 2007 (the latest data they seem to have). Let's assume 25% out of 6 billion, which is 1.5 billion.
So we can deduce that one in 15 internet users has the Alexa toolbar installed, hence
P(A) = 1/15.
I seem to recall (but correct me if I'm wrong) that on some Podcast, Joel and Jeff mentioned that they estimate SO's worldwide potential audience size somewhere between 4 and 8 millions. Let's assume 6. So 6m out of the 1.5b internet users are (potential) Stack Overflow audience,
P(S) = 6 / 1500 = 0.4%
The Alexa page you refer to gives SO's reach as about 0.2%, in other words, the probability that a user that has the Alexa toolbar installed is a SO visitor is
P(A|S) = 0.2%.
According to Bayes' theorem,
P(A|S) P(S) 0.2% * 0.4%
P(S|A) = ----------- = ----------- = 0.012%
So, 0.012% of the 6 millions SO audience, or 720 people, have the Alexa toolbar installed. Because of the technical background of the people here, who are a lot less probable to install a spyware addon into their browser, the actual number is probably less.
According to a reliable expert, 14% of people know that people can come up with statistics to prove anything. Thus, beyond any reasonable doubt, I have proven that the Earth is a disc. Now what was your question again?