"Do the majority of the brands using Taggo aim for engagement over fan growth?"
Yes, that's probably an accurate assessment. Right now, the companies that are showing the most interest in Taggo are those that want more interaction and engagement with fans, especially with fans that actually shop in their stores. As this activity matures, brands will become more interested in measuring active fans versus the total fan base.
I don't think the metrics will become exactly like those used by the credit card or telco industries, but some of the metrics should be very similar.
We track the number of new fans generated during a Taggo campaign, but I have to admit that I'm paying less and less attention to that number. I'm watching numbers like check-ins by fans and friend activity such as clicks on posts generated by Taggo, and comments and likes on those posts. All a learning process :)