You intentional blurred the normally clear distinction between something sold (product/service), and a buyer. In the case of Google, Facebook, LinkedIn, Twitter, and similar, the users of the service are the product being sold, and the buyers are (typically) advertisers. The users are given something useful and beneficial (search results, creepily stalking other people, pointless "business" connections, publicly broadcasting the precise color of your last bowl movement, etc.) to attract them, but there must be someone else, a buyer/customer, willing to pay for it all or you don't have a business.
In the above sense, collecting users by offering free services is really no different than manufacturing physical products; in both cases your expending capital to create a "product" with the intent of selling it for more than what it cost you to create.