RapLeaf offers a service where you give it an email address and it returns the Facebook, Twitter and other social media accounts associated with it.

Now, I'm not saying they are using this vector, but then they must be using something like this because how else could they offer the service. (This also means there might be other vectors to achieve this end result).

To me, this also makes me pleased that I use a unique email address against my email domain for each site I use.