Peer-to-Peer (P2P) file sharing applications have witnessed a dramatic increase in popularity during the past few years. To accommodate the rapid growth in user population, developers introduced new features in their client software, in particular a two-tier overlay topology. The effect of the two- tier overlay topology in a widely-deployed P2P system primarily depends on the availability and coherency of its implementations among participating clients throughout the system. This paper sheds some light on the long-term evolution of such a two-tier overlay topology in the Gnutella network during a 15-month period over which the system quadrupled in size, exceeding three million concurrent peers. Our results show two interesting phenomena including: (i) During this period, the two- tier overlay has repeatedly begun to lose its balance. However, proper modifications in major client software coupled with the rapid upgrade rate of users, has enabled the developers to maintain the overlay's desired properties, (ii) Despite its random connectivity, the Gnutella overlay exhibits a strong bias towards intra-continent connectivity, especially in continents with smaller user populations, that has not changed as the system scaled.