0%. RoR wasn't released when we released Basecamp, our first product. RoR wasn't really a thing yet.
I'm sort of curious how much of the growth of basecamp you attribute to the RoR community though? Do you think if you had instead just written it with PHP or something you would have had a similar level of success?
Unless you are specifically targeting some niche market of software engineers, the underlying technology just doesn't enter into the discussion. Basecamp was built for project managers.
If you do a good job impressing them, they will help spread the word.
Rails was assisted by 37signals' already established brand and name. The blog already had tens of thousands of subscribers at the point it was released.