You're not being fair to IRC. IRC can just as easily give you a one-to-one relationship with another person who cares. Private messages exist, among other means. Moreover, the experience can be completely asynchronous. You don't have to both be available to make progress.
The main benefit of something like Understudy is to mediate the introduction of users to mentors. There is something to gain from a sustained conversation with the same mentor, but Understudy can make no more guarantees about the availability of your mentor than IRC can. Furthermore, that both the mentor and the mentee have to pay $5 if they're using the app more than two hours a month demolishes the potential userbase, not to mention that you also need an iOS device. IRC is multi-platform, free, and already attracts the sort of individuals that would be capable and willing to mentor users in something like SICP.