Measure how your impressions spread on networks. Are your posts noticeable ? How many people actually saw them ? Pample answers those questions.
Do your fans visit your page ? How does this metric evolve overtime ?
Compare your organic and paid impressions ; determine how your investments perform in terms of content distribution and fan generation.
What happens where your content is shared ? Does it attract more people to start engaging with your page ? Measure how many people interacted after a content was shared.