The only real complaint about this purchase would be that LinkedIn was overvalued. And I wouldn't bet on that since Microsoft surely sent a big team of their brightest cookies to make a valuation of the company.
But other than that, LinkedIn fits: it is a corporate social network, if you will. I know that "corporate social network" is almost an oxymoron, but bear with me. It is social in the sense that users register and interact with each other just as on a regular social network, but ultimately the service is corporate focused. LinkedIn has created one of the biggest and best maintained databases of employment information (and of possible business contacts) in the world. As TheArcane said, LinkedIn is "Facebook + Monster on steroids".
If you don't believe me, try to speak to people you know, active professionals outside of the IT sector and see how many of them have LinkedIn profiles. Better yet, talk to people who are not in employer-dominated sectors (i.e. not like IT, where the demand for qualified workers greatly exceeds the supply).