> But how do they differentiate between the two. How do they know, that endorsement a is less worth, than say endorsement b?

E.g. they know _when_ a user endorsed, and if they endorsed all four or individually. And they know more about the user--ie how often he endorses, and who he's connected to.

And, yes, I agree with your low opinion of linkedIn and co. (Even though a close friend of mine actually got a job with the MathWorks in Germany through linkedIn---she was cold-emailled by an internal MathWorks recruiter.)