Considering they had a design business before they started selling online tools, I'd say not as high as you think.
In fact I'd make a guess that most of the 'beta testers'/first users of Basecamp were probably former clients that had projects to manage.
Rails developers probably helped with the explosive growth of the products, but not as much with the initial investments. Keep in mind the entire idea of Basecamp was that it was to be an internal tool.
Yes, the explosive growth was what I was getting at; I should have specified their first paying customers.
It's kind of like wondering how many customer Fog Creek software got because of joelonsoftware.com or indeed how often YC gets first pick of the best startups because of HN.
In fact I'd make a guess that most of the 'beta testers'/first users of Basecamp were probably former clients that had projects to manage.
Rails developers probably helped with the explosive growth of the products, but not as much with the initial investments. Keep in mind the entire idea of Basecamp was that it was to be an internal tool.