Several years ago, I saw an article or two (I can't find them now) along the lines of "Google knew I was pregnant before I did." The upshot being that a woman or women suddenly started getting served Google ads for strollers, disposable diapers, etc., and then a week or two later had occasion to pee on a stick and see a plus sign.
In its right sidebar, Twitter suggests accounts to follow. Some of them are organic (people who follow, or are followed by, some of the same people as you, for example). Some are "promoted," meaning they're ads.
So anyway, I just saw this in my Twitter sidebar: