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.

Good question. Not to be a pessimist, but in most scenarios I have found partnerships to under perform initial expectations. Part of that can be mitigated with the things I outline in the post. But I think more than anything finding that one key predictor of success is important. For Viximo it was sites that had high DAU/MAU ratio. We'd rather take a company with 1/10 of the total audience, a crappier experience, and worse business terms but a high DAU/MAU ratio. It made a huge difference.

But that key predictor is different for every business. Finding it can be tough. But once you find it you increase partnership success chances by a lot.