LinkedIn is much more business relationship focused vs. Facebook being more social relationships (IME/O). I get far more "spam" on Facebook than I do on LinkedIn.
If I haven't actaully interacted with a person in a business context I typically do not connect with them in LinkedIn. In Facebook it is people I know in the real world but there may not be a business relationship. I don't link with people I do not know which keeps true spam to a minimum.