The population growth of Splinterlands seems to be following a positive feedback loop based around Hive. People spread the word about Splinterlands and get rewarded SPT. The rewards can be exchanged back into the game promoting itself. At a glance it seems like a ponzi scheme where the game relies on new players adding money into the game. This isn't the case though, because the game can thrive off existing members spending, playing, and socializing. Splinterlands will depend on keeping a core playerbase and not people looking to make a quick buck.