I think it's more likely that Vayable just "grew at the same rate", where "rate" should really be "amount".
30% of 70 is 21 and April 2011 is about 86 weeks ago. 21*86 = 1806, so a rough projection of 1876 guides. The vayable website promises "2000+ tours", which with roughly that amount of guides with that projection actually makes sense :)
I'm driving down to Mexico City through Guadalajara in late December - just signed up with Vayable and am thrilled to find a couple tours I hope to take on the trip.
My wife and I found some awesome tours/guides in Puerto Rico on Vayable when we were there. Couldn't actually do a tour because it was literally last moment, but I plan to check Vayable before each vacation now when I book my hotel and flights!
30% growth every week for 18 months would mean total growth by nearly a factor of a trillion. This doesn't seem likely.