Recruiters are generally random and have no qualms about adding people.
People also will add people at the company, even if they don't work together, or even colleague of colleague type connections are pretty standard. Doing so on FB I think would be weird, especially across genders.
To me this makes LinkedIn pointless. The whole point, at least in the beginning, was to create a network of people you've worked with.
If you start adding random people it becomes like yet another social network without any added benefit.