I have one pet peeve about this, which is typically my pet peeve on a lot of growth hacking / optimization writeups: lack of longer-term focus. I hate to preach too much because doing things without a proper measurement strategy may be more practical than implementing a proper measurement strategy and is better than doing nothing at all, but....
The results were listed as "very good" because 20% of new signups tweeted. However, you're giving away revenue in hopes of generating more long-term revenue. The success or failure of the experiment should be given in terms of customers who signed up from seeing the tweets and what the lifetime value of those customers are, taking into consideration the initial revenue loss.
You're right. What's important isn't how many people tweet, it's how many people sign up. Of course 20% of new signups will tweet, they get a free month for it.
That's why you give a user-specific shortened link with UTM values for people to tweet out and don't simply reward them for tweeting your site name or address.
The product is valued at "a tweet for a free month". Does this support the pricing tier? There's something about the perception of price and brand value here.
I disagree. I'd say the free month is not for a tweet, but for the number of users who sign up because of your tweet and then how much those users end up paying to use the product. If just one user signs up because of your tweet and pays to use the product for a month, then what's been given away has been earned back. And none of this says how much the product should be valued quantitatively, i.e. how much one month is worth.
On the other hand, you do comment about perception–if the users perceive the product to worth less because of this deal, "a tweet for a free month", the rationale behind why the deal is good for the product might not do anything to offset that loss in perceived value.
Your assumption, however, is that people wouldn't tweet about it if they had no incentive. The question now is: Does the tweet incentive help to increase the user base or not? That would actually be interesting data :)
The results were listed as "very good" because 20% of new signups tweeted. However, you're giving away revenue in hopes of generating more long-term revenue. The success or failure of the experiment should be given in terms of customers who signed up from seeing the tweets and what the lifetime value of those customers are, taking into consideration the initial revenue loss.