AdAge ran an article on LinkedIn today that was both obvious and enlightening. The obvious part is this: while Facebook’s popularity has snuffed out other social networks (RIP Friendster, Orkut, etc.), LinkedIn is flourishing-to the tune of 85 million members, with another joining every second.

The enlightening part is how LinkedIn’s success emphasizes the importance of knowing what you aren’t. LinkedIn remains popular despite Facebook because it remains committed to a mission that is very different from Facebook’s. While Facebook connects friends and exudes a casual vibe, LinkedIn has stayed focused on the narrower, more buttoned-up task of connecting business contacts with each other.

The success started with LinkedIn seeing clearly what it was (a business networking forum), and what it isn’t (anything else).

It did not have to be that way. In the race among social networks to grab users, LinkedIn could easily have expanded its mission, adding features like picture sharing or games to attract a wider audience not necessarily interested in professional networking.

But it didn’t. LinkedIn’s refusal to expand its mission requires remarkable restraint, and leads to observations like the following: “[LinkedIn CEO Jeff] Weiner is adamant the users on LinkedIn aren’t there to share their personal lives with each other.” Mr. Weiner, in fact, doesn’t even like referring to LinkedIn as a “social” network. (We’re afraid that ship has sailed, but the sentiment is notable.)

Certainly, it costs LinkedIn members to remain a business-only environment, at least in the short term. But that focus is also what gives it an identity entirely separate from Facebook’s, and why LinkedIn is able to thrive alongside the Facebook behemoth. It’s the reason that if professionals are going to be social networking, they need to be on LinkedIn.

Of course, the execution goes hand in hand with the vision. LinkedIn doesn’t just know what it is, it also provides solid tools for professionals to market themselves. LinkedIn keeps its features and applications very limited, but those it does offer-recommendations, Q&As, JD Supra distribution for legal writings, etc. – allow its users to create profiles that make powerful statements about who they are professionally. LinkedIn’s success in differentiating itself as a professional network-through its vision and structure-is why all professionals looking to network need to be on LinkedIn, despite the presence of Facebook.

The success started with LinkedIn seeing clearly what it was (a business networking forum), and what it isn’t (anything else).

Is your firm like LinkedIn? Can you identify a piece of business that your firm would turn down because it’s not what you do? If you can’t, you need to think harder about what you aren’t. That will help you find out what you are.