Difficult question, as it depends how success is defined. But as an advertiser, after identifying several targeted networks in my vertical, I would look at the following types of metrics:
- average time to 10-th most recent visitor
- average time to 50-th most recent visitor
- average time to 10-th most recent NEW visitor
Also the advantages of these metrics is that they can be computed easily. An advertiser can run a little web crawler to automatically compute these metrics on target publishers (networks), rather than relying on 3rd party statistics that can be inaccurate. For instance, on AnalyticBridge, time to 5-th most recent NEW visitor is about 12 hours as of today. You can just look at the front page to manually compute the number (a web crawler would automate the computation and also compute averages over several days).
These are important metrics to get an idea of reach and frequency. Of course, would such metrics become popular, you would start seing some networks inflating they member counts by signing up fake members. But we are not there yet.