Could not agree more. Although retention is indeed very hard to measure, and things like net promoter score seems a bit flaky at best.
I am curious, Does anyone use net promoter score extensively and found it useful?
I think twitter could potentially be pretty effective if you combine the number of mentions with a sentiment score. But then, it's pretty easy to increase twitter engagement using various tools.
I'm also looking for a good way to measure net promoter, but for another reason -- to quantify it for investors. We hear great customer feedback and get referrals all the time, but don't have a measurement for it. Even though it's true, I'm not sure investors will just believe that "our customers love us!".
If you want to measure customer loyalty and quantify it for investors, you can use that methodology, or you can find another way but call it something else. I wouldn't recommend doing something else and trying to call it NPS.
In addition to Net Promoter score, consider tracking K-Factor for virality, which is way more objective, and might give you some hints on what your NPS score is.