From gov.uk on the same topic:
It’s fair to say that the introduction of sharing buttons didn’t exactly set the world alight. Overall sharing rates using the buttons are pretty low, way less than 1 percent.
However, there is evidence that social sharing buttons can be substantially more useful for people using mobile phones than on desktops; and - as you might guess - that some types of content are more popular than others for sharing.
Overall, our social sharing rate was 0.2%