LinkedIn has instituted 2 Factor Authentication

Not a lot to say here. 

LinkedIn has implemented 2FA (2 Factor Authentication). This makes logging into LinkedIn more secure. The setup process is very easy, so if you haven't set it up, already, please do so.

If you run a website that requires users to login for important transactions, then you should implement it, too and get yourself on to this list. More information on 2FA can be found here.