Ways to keep it going:
Add trading for more social engagement (already planned). Add pvp battling, as well. Award EXP and items for both (balls, potions, revives, etc).
Create buzz with "legendary raids" requiring teamwork at the same place and time (like the bit in the live-action trailer where everyone fights Mewtwo at NYC Times Square).
Phase in more Pokemon: Keep the original 150 constant, but rotate in subsequent gens alongside them, in seasonal waves. Example: The month of August = Gen 1 + Gen 2. The month of September = Gen 1 + Gen 3. The month of October = Gen 1 + Gen 4. And so on.
More Pokemon = more variety, less duplicates. To balance for less duplicates, lower the cost of evolution (less candies and stardust), and increase number of candies received for catching and transferring Pokemon.
Add shiny Pokemon for more social media buzz when people post their rare finds. Add gender differences for yet more variety.
Pokemon Amie: Pet and feed Pokemon to build affection and bond, which lowers the cost of powering up and evolving.
Add Facebook, Twitter and Instagram integration, in app, so people can post screenshots directly to social media.
...there are all kinds of ways they can maintain interest.