> The problem is that it's pretty rare that you re-convert a churned user.

Pretty sure this isn't true. Reconverting a former customer is cheaper than converting new customers from an advertising perspective. I wouldn't imagine that social media customers are any different.

I will say that it is generally cheaper to keep an existing customer than to re-covert them later, but that's a separate issue.

I imagine the case here is that people downloaded threads to check it out, and then stopped using it because the network wan't as large as Twitters'.