How Twitter Knows So Much About You Before You've Even Signed Up
The truth is that learning to use Twitter was really hard back then, and many new users abandoned the service. In 2009, its retention rate was worse than MySpace, threatening its ability to grow to critical mass. Too many new users were simply too confused to stick around and learn how to read and write 140-character posts.
It got over that. Twitter now has more than 200 million active monthly users. And a key reason why is its slick process for onboarding users.
One big reason why people abandoned the service was that they couldn't figure out who to follow on Twitter. So in 2009, the information network started with a more or less hand-picked "suggested user list"—the same for everyone, heavily weighted toward Silicon Valley figures and Hollywood celebrities. The following year, it started tracking patterns in who followed whom, and used that to target suggestions.
But in 2012, Twitter took a big step that's still not well understood. Publishers have placed Twitter buttons on millions of websites, making it easy to share pages on Twitter.
Even if you don't tweet on a page—in fact, even if you don't have a Twitter account—those buttons allow Twitter to track which websites you visit, if you haven't or the publisher hasn't opted out.
The website-visit data is deleted after 10 days, and Twitter makes it easy to opt out. (It's not collected from European visitors, because of local laws).
But this is a primary way Twitter has improved the way it signs new users up.