> It would be interesting to run a semi-controlled experiment to test whether this was a fluke, or if they leaked, sold, or otherwise lost control of my data.

Too much time / energy on your hands? You gave them a unique email ID (which is always the most sensible thing), that's it.

The non-sensible thing was to sign up kn the first place. Nobody needs these narcisstic, BS spewing pseudo-networking places.