LinkedIn pulled something similar a few years back. At the time, I was using the same password for both my email and LinkedIn account, and found that people from my email address book were showing up as suggested connections. I can only assume "consent" for this was buried in the T&Cs.
In case of Linkedin, they do ask for consent. But, even if you didn't allow it to export your contacts, other people may have allowed it, linking their linkedin profile with your email address. LinkedIn then shows them to you as suggested connections.
This personally bothered me for a while then.
