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Have you used it on mobile and shared your contacts? Might they have had access to your location(GPS or maybe to which wifi you're connected).

Or they just suggest people based on common locations that you and they visit and where they spend their time.

The mobile app's contacts seems very likely. I didn't realize it would spy on those, but not surprising.

The people who were recommended to you might have looked at your profile.

You've probably shared an IP address (work).

That one is highly unlikely, as I work remotely