This is Microsoft's biggest acquisition to date, and their track record hasn't be great, generally overpaying and under-delivering. They are Skype's third owner, and you'd be right to question why this name has taken over from Lync, but clearly, they're sticking to it. MSFT paid a similar amount for Nokia, which was written off about a year ago, and a few years before that their pricy acquisition of aQuantive met a similar fate.
They have never been able to crack the mobile market, so precedent does not suggest Linkedin will be a home run. For that much money, it better be, but on the other hand, MSFT can afford it. Perhaps more importantly, they can't afford to miss big opportunities, especially in the business market, which is easier to monetize than the consumer space.
Linkedin reportedly reaches an audience that's bigger than Skype - some 430 million vs. some 300 million - and they've built a highly engaged community with solid business value, whereas Skype remains largely a low cost or free consumer-grade communications platform. No idea what the synergies may be between those two worlds, and it sure will be interesting to see how it unfolds.
Of course, there are concerns that the Linkedin community will become a sales channel for MSFT products, and if it's not handled right, another pureplay business/professional network platform will surely emerge to give that community an independent alternative. Lots of angles to explore, and there's more on the podcast, so I hope you give it a listen.