Most advanced paywall companies have a feature that enable premium content publishers to make exceptions to the paywall for referral links. Yes, they want subscribers. But they still want to drive traffic from social sharing.
Tinypass does exactly this with its "Counter" feature that displays to readers how many free previews they have left in the metered paywall. Then, when the meter expires, the content is blocked or a donation is requested. Hundreds of publishers are using the platform. http://tinypass.com