I’ve been looking for years. A beehive! An example in nature of a two sided network effect! It provides brokage, exchange, or agency between two classes (pistil and stamen). It draws off a tax (honey) for the maintainance of the hub (hive). Since these kinds of hubs become more valuable as more and more particpants rondevous around the standards set by the hub I think we can assume that the flowers of the client population evolve to fit the exchange standards made manifest in the hub. I.e. they evolve to interface well with bees of a certain size et. al. of the dominate bee population. Overtime that should drive other polinators out of buisness.