Saw the title and thought it might be you two. The site looks great! I'm surprised you found the time to put something like that together while in YCombinator.
Of course I have to suggest Python/Django, since that's what my company uses. But to maximize (number of users)x(number of security vulnerabilities), perhaps Wordpress plugins would be worth monitoring?
I've put something together over the last weekend, tailored for django. It's still relatively early, but if you want to check it out: https://github.com/jayfk/django-updater
Of course I have to suggest Python/Django, since that's what my company uses. But to maximize (number of users)x(number of security vulnerabilities), perhaps Wordpress plugins would be worth monitoring?