They mention it in the article:

> None of the proposals were as lucrative as Facebook’s ad-based model. “Well, that doesn’t scale,” Ms. Sandberg told the WhatsApp executives of their proposals, according to a person familiar with the matter.

I think it's partly a self-selection problem, where the companies which use the service don't have enough revenue to contact their customers through dedicated channels. If you try to monetize this, then most companies just leave because those who could pay already have such channels to customers.

It's the same problem with offering users to pay for an ad and tracking free facebook. Those users who would pay for facebook are exactly the ones you want to show ads to, whereas the others are uninteresting revenue wise either way.