That's a tricky one. The challenge in my mind is, there's what you want to measure and what is easy to measure. Depending on your goals, you likely want to measure things like engagement, influence, or the 'viral' nature of your campaign. However, we all know it's far easier to measure impression, visitors, etc.
Can I ask, what are your goals? And has your agency offered either metrics or pricing models that are more 'social' and less focused on counting passive eyeballs?