Do you have plans for bots / bot networks that can simulate a conversation between two or more people? I do think the overall premise is interesting though and can see how it might deter undesired content from taking over your social network.
There's certainly a risk that some kinds of abuse patterns will get through our reply-to-publish model. Eventually we'd like to try and detect bot traffic and challenge them with a CAPTCHA or similar.
I'm not a rocket scientist, but my understanding is that efficiency is derived from the exit velocity of matter from the engine. So the faster it can fire matter in the opposite direction the more efficient it is. There are some other factors as well like bell shape, which are generally designed to provide better efficiencies at different atmospheric pressures (ranging from sea level to vacuum of space). The main consideration after these factors is the mass of the engines. If multiple engines and one giant engine had all of these same factors equal, then I don't think there would be much difference in the efficiency of the rocket. Likely though, there are differences and other advantages to multiple small engines. The easiest one for me to think of is modularity. If you want a specific thrust for a payload it's much easier to add more small engines together than to try and throttle down the large engine to the correct amount, if that is possible at all.
On a related note for pens, we recently tested through a series of numerous thin-tipped gel pens and found that the Pilot Juice Up 0.3 had the smoothest writing and best combination of ink visibility and lack of smearing (due to later water damage).
I think the 2.6 million books refers to 2.6 million different titles that were sold rather than the total number of book sales across all titles, which as you pointed out is significantly higher than 2.6 million.
