The official Yammer blog has an interesting post asking "how much adoption is enough" for enterprise social networks. The nifty charts define the last 16% of the workforce as "laggards"/"naysayers" who might never adopt a social network, and so pouring resources into getting them into the network is likely a futile task that drags down the ROI for your strategy as a whole.
The most valuable insight comes about halfway down the page: "Keep focusing on the people who drive value out of the solution and the others will eventually catch on." Indeed, it is crucial to focus on value when trying to convert users. They won't sign up -- and come to habitually use -- something like Yammer because of the features or potential that the tool has. Many of them will sign up if an executive urges them to do so, but they won't become steady users of the service for that reason alone. If they see value in using Yammer -- value that exceeds the potential value of the time spent doing so -- they'll use it. If they don't, they won't.
That being said, be careful not to focus too tightly on one particular form of value. Value varies across users. Innovators find value in the new, latest thing because it's new and cool. Highly social users find value in social media because it allows them to socialize without leaving their desks. Utility-focused users find value in Yammer because it's a quick and effective way of asking questions and getting answers. Others will find value in the ability to keep better informed than they would be otherwise, or tune into certain people and topics (and tune out of others).
Others will see no value at all, and they may be right. Even if they're wrong, it's too much trouble trying to convince them to change their mind. Better to focus on supporting and enhancing the value that regular users do find in the service; their success is ultimately the best argument that you can make.