That's completely unfounded speculation, and I'd love to see any citation for it.
I'm inherently skeptical because for these companies their most important asset is their data. The idea that they would share that data willy-nilly with third parties makes no business sense.
http://www.interactually.com/linkedin-creepiest-social-netwo...
http://imonlinkedinnowwhat.com/2009/09/14/linkedin-people-yo...
there are quite a few instances of weird guess about the people you might know suggestions
of course the actual source is kept as one of their best secrets.
The far more intelligent explanation is that suggestions are, as LinkedIn repeatedly emphasizes, based on imported contacts. Note that even if you don't import contacts, the other person might have.
Try using Occam's razor before jumping to unprofitable conspiracy theories. (Again, why on earth do you think Facebook would voluntarily share data with Facebook?)
Because why not? Facebook isn't really competing with LinkedIn right now - the latter doesn't try to enter private life, and the former doesn't really touch work life much. Both could benefit from sharing data with each other - from better recommendations to creation of shadow profiles. I don't see how this is an unreasonable theory.
I had used a chat service only once to call some guys and the day after all participants were suggestions on linkedin.
To most of those free service a product is you and they monetize your graph in this way.
Then you have subsidiaries. You may gave not given facebook or messanger access to you contacts, but what anout whatsapp? Any app with access to your contacts may have sold them. And even if you have none the other party might, or even a third party.