Not surprising really.
A few years back Hotmail/Outlook were returning people's Twitter/LinkedIn handles for emails sent/received. It had been noticed you could scrape that fairly easily at scale. With one email account you could check up to 30000 email addresses before being flagged by Outlook.
Slightly longer ago you could simply iterate 1...n on LinkedIn URLs to find someone's profile, by converting the number to base12, you'd be redirected to the person's public URL.
Also their bulk contact upload. Take any data leak of email addresses, bulk upload them as contacts and then correlate email addresses to social profiles.
Facebook, Twitter and LinkedIn are all bad in that regard on the last method, though Facebook at least do not return people's URLs along with your contact upload (you're expected to know the person's face/name to decide whether you'd want to connect). The take away is that once you sign up, whatever information you put on your profile/account is pretty much available to anyone who wants it enough - and clearly there are plenty bad actors who want it. Obviously these social networks want to expand their network, but they also make it much more easy for data harvesting at unprecedented scale.