Facebook’s product is not a social networking platform or service. Its product is its users, which it sells to advertisers. If it hadn’t built up its user base first, it would have had nothing to sell to the eventual advertisers it took on. You’re right to suggest it’s foolish to build a user base without thinking about how to make money, but in the case of Facebook and other networking platforms (Twitter, Kik etc.), there’s really no way to make money without first establishing a strong and growing user base, because the user base is the product.