One thing to point out in this specific case is that the demographic to which SocialSci directly benefits (i.e., survey participants) resonates with a (rather large) subset of Reddit users, contributing to their surge in sign-ups.

I'd even go as far as saying this kind of "organic" discovery by a more diverse community trumps the spike in traffic generated by being frontpaged on HN, whose user base is admittedly a little more technically minded and bourgeois in nature, and consequently reflects poorly on long term growth potential.