How to find out if any of your contacts are online?

How does LinkedIn find obscure contacts of mine to recommend to me to add to my network?

• I have never given LinkedIn my Google contacts. From those obscure contacts sharing their Google contacts? Or does Google "sell" contacts to Linkedin?

• Answer:

  Chances are that those people whom LinkedIn is recommending to you gave access to their Google contacts in order to look up potential contacts already on LinkedIn. Evidently, one of the addresses in their contacts belongs to you, hence the suggestion. It's worth pointing out that, when you authorise one service (in this case, Google) to allow access by another service (in this case, LinkedIn) to part or all of your data held by the first service, that authorisation is forever, or until such time as you go in and revoke the authorisation. This applies to all the major social networks. Chances are that you, dear reader, authorised Facebook to log you into Quora. The good news is that it's fairly straightforward to find all those authorisations and revoke them if necessary. The bad news is that you may find a very long list awaiting you...