Sounds like genuine interest is worth more than number of followers. The question is, how do you discover users who would have genuine interest and would share with like-minded friends? hashtag searches?
Analyze follower of followers once you've identified a few users who seem like potential fits for your space. I've seen this strategy work well in other social channels as well like twitter.
Step 2: Look at who actually follows them. Are they real people? Are they bots?
Step 3: Look at how many products they promote through all their products. The more the worse.