I'm not sure at all that it is clear what the nature of that link is. I have no insight into the gmail's email relationship modeling but I suspect that it doesn't automatically build a relationship between [email protected] and other people that onion2k contacted. For one thing, it would be impractical to do this across all possible relationships -- there would be polynomial growth of the adjacency matrix with no information gain to justify it. Ask yourself, what does it mean that david contacted onion2k and onion2k, at some point in time, contacted alice, bob, and charlie? Unless you already know something nontrivial about these people, you assume they are independent.
I guess my point is that data analysis can be very costly; it is reason that humans have a hard time dealing with meaningful relationships spanning more than about 150 people[1]. Even a company with Google's resources would be overwhelmed if it had to store and take into account a large number of mostly (I mean, vastly mostly) meaningless relationships. At the least, it takes away resources from the relationship graphs that do matter and which are actionable, such as the graphs of spammers.
But does Google profiles it?
For example I use [email protected] to send emails to [email protected], [email protected] and [email protected]. Google now have a profile for [email protected] with a graph that includes nodes for alice@, bob@ and charlie@ and edges for the relationships between the four people, and links to anything we've discussed. If [email protected] then sends an email to [email protected] Google know that david@ has a second order link to alice@, bob@ and charlie@ despite the fact that they have never communicated or informed Google of this relationship, because Google have a profile based on [email protected] - an address that Google might not have a justifiable reason to be building a profile on.
Whether or not you believe that is a reasonable use of data is up to you. Some people think it's not.