Sirvo has been fairly quiet about it up until now. Today they've released a huge article on the development of Threes featuring the 570 emails that the team sent to each other during that process. They explain how the concept was done quite fast but how they struggled with the mechanics, and much, much more. From a monster that was eating the tiles to the now popular "doubling" gameplay that was added 7 months after they started making it, you'll be able to have an in-depth look at how they made Threes and at how difficult it is to make a game that feels so simple. I really recommend that you do read it, because it's fascinating.
In any case, 2048 (and 1024 before it) perfectly illustrate how quickly clones can take over the App Store. Or, as the Threes developers put it, "We do believe imitation is the greatest form of flattery, but ideally the imitation happens after we've had time to descend slowly from the peak -- not the moment we plant the flag."