

How does Google know what other accounts might be yours? - joejohnson
http://www.google.com/support/websearch/bin/answer.py?hl=en&answer=1142745

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jeggers5
Google is basically a massive brain - the know everything about the internet.

I'm sure they have lot's of methods of figuring out who's who on the Internet.
They know a huge amount about you (well they try to anyway).

Besides, they've got my LinkedIn A/c, Facebook A/c, Twitter A/c all linked to
my G+ Profile now because I told them.

