so it's not you being watched, it's Mark and Chipps.
Oh f...... I risked a shower this morning with the blinds open too.
It's Google ads btw that does the contextual ad serving. It's fairly straightforward and not quite as insidious as it sounds. If you click on 'View' on your browser menu and then 'View Source' you get a complicated page full of plain text. This is the code that makes up this page. In that text are all the words wot we is writing.. Google Ads just trawl through that text and match up key phrases with ad campaigns that advertisers have paid for that include those same key phrases.
We sometimes use it ourselves to advertise the mag and website. I'll think up some ad using our Google Adwords account and then I'll select a big bunch of key phrases I want our ad to look for on the internet. When Google Adwords enabled websites publish those key phrases our ad 'could' be served on that page.
I say 'could' because for popular phrases, like 'MTB' obviously lots of advertisers will be selecting that as a target phrase. When there is competition for a certain word or phrase then a bidding war ensues and we have to set a price we are willing to pay to have that phrase associated with our ad. If we top the bidding table then our ad will be shown. 'Cycling' is a rather expensive word it turns out. Luckily 'Singletrack' is cheaper
So, you see, it works without prying into anything more personal than the words you type in a post or the words we include in the page. It does not associate anything with any personal information on an individual level.
The result is ads that are more targeted, which is actually more useful. Depends how cynical you are as to what your opinion of the whole system is. personally I giggle a bit when the Fitness Singles ad keeps popping up, or the ad for RC helicopters when someone mentions 'helicopter tape'