There are all the obvious public metrics from things like Compete, Alexa, app rankings, twitter following, facebook following, etc etc.
But beyond that I would dig deep into using the product. If I was going to assess the engagement (DAU/MAU) of a social network I would look at how people are commenting. Or maybe send 20 friend requests/messages to random people and see how fast on average they respond. You can get a feel for certain things just by using the product.
Otherwise I would just reach out and start a discussion. Some partners will be shy to give up metrics, but you usually can pull it out of them over time.
But beyond that I would dig deep into using the product. If I was going to assess the engagement (DAU/MAU) of a social network I would look at how people are commenting. Or maybe send 20 friend requests/messages to random people and see how fast on average they respond. You can get a feel for certain things just by using the product.
Otherwise I would just reach out and start a discussion. Some partners will be shy to give up metrics, but you usually can pull it out of them over time.