If your product is truly viral within its core user base those users will tweet and talk about the product anyway.
It would be nice if iubenda does something similar a few months after they exit private beta to show us what numbers are real and what is just success theater.
About what's real, I think that it's mostly the need. I read every tweet that comes on the "iubenda" search, and frequently look to the profile of the user behind. Most of the times the user is a web designer, which proves that people subscribe because they feel the need. Moreover, I interview almost every beta tester that we invite, and again I get people really passionate about the product.
I think that the marketing-driven risk is mitigated by the fact that the product is explained very well on the website, and people signup only if in need of the product.