We use Qualaroo for NPS in our community. There is a ton of flexibility in terms of targeting. For example, http://help.qualaroo.com/hc/en-us/articles/201441516 lets you do pretty much anything. You can target based on scroll depth, # of pageviews, Optimizely experiment/variation etc. We have it deeply integrated with Salesforce, so it works well for us.
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