I actually had almost exactly this in the original version of the article. I changed it because there are lots of products/solutions that don't require people to pay for them but where the same framework still applies -- Google, Facebook, and LinkedIn are three examples.

I think you had it right the first time, and changing it detracted from your real point, "Why now?"

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.

Facebook, Twitter, etc. also have to build something that their users want to use and this follows the same paradigm. Google could build something to make every advertiser happy but if it pissed off all of their users it would kill their business. They have to solve this problem on both sides -- they have to build something users want to use and they have to build something advertisers will pay for. Both halves of it have to answer the Why Now