I think that Bitcoin is as big as I think it is - a $2B economy with >10K businesses and hundreds of thousands of users.
Based on data from
http://www.similarweb.com/website/bitcointalk.org, it looks like the forum gets about 1M pageviews per day.
Google:
how do I buy
look at what comes up
You do know that Google search results and autocomplete are targeted? If you look at Bitcoin all day, it will give you Bitcoin-related results... It has little to do with Bitcoin's popularity.
AFAIK targeting is done only for language, location... and previous searches only if you allow Google to keep your search history.
Dont be so naive, they also use your gmail content, G+ content too, they are even able to differentiate different user from the same IP.. Quite a lot of researsh have been made from them about it, and they use a lot of trick to target/track user.. Do you know each page you visit that contains a G+ / FaceeBook / Twitter / Linked / .. share button are relaying your IP to those site.. Dont beleive they wont use this data !
So they all know your IP visited this page. I bet there is something similar with GoogleAds..
Then I also use ShareMeNot
http://sharemenot.cs.washington.edu/