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Reid Hoffman is the co-founder of LinkedIn and an investor in Facebook. Why isn't that a conflict? Because he doesn't consider the two competitors. User base and functionality aside, a lot of it boils down to business model. LinkedIn is one of the only Web 2.0 companies that charges for subscriptions and premium services in addition to selling advertising. And while LinkedIn and Facebook are both doing about $100 million a year in revenue, Facebook has largely done that by outsourcing advertising to Microsoft, while LinkedIn has built its own sales force from the ground up. Hoffman explains why LinkedIn has always had a culture of picking solid steady growth, over sex appeal. And, he weighs in on Facebook's $15 billion valuation. Plus: How big LinkedIn's market can get.