That was pretty much the Yammer sales strategy, as I observed it. Infiltrate the customer by making it free and easy to sign up an organization, build a user base organically, without having to involve those controlling the purse-strings. Eventually, there's enough people using it that compliance and security becomes an issue, and the business sort of has to pay for the enterprise support to CYA.
Credit where credit due - I think Basecamp were one of the early ones to perfect this strategy.
When something is low enough to experiment with and still fall into the "petty cash" area, an employee can buy a little (or for a month) and test it out. If they're somewhat successful, they can expand it out to their immediate team, still with a tiny budget. As they prove out usefulness/value and expand their users/usage, obviously the cost goes up.
But by the time they have to "Get Approval" from someone really high up - like a VP or procurement group - they have X users or Y months of success. Selling it to the higher ups then becomes easier and a more likely sale.