Some twenty years ago, Paul Geraghty offered a large-scale survey of the retention and loss of Proto-Oceanic *R across Eastern Oceanic languages, and concluded that *R was “lost in proportion to distance from Western Oceanic.” This paper aims at testing Geraghty’s hypothesis based on a larger body of data now available, with a primary focus on a tightly knit set of languages spoken in Vanuatu. By observing the dialectology of individual lexical items in this region, I show that the boundaries between languages retaining vs. losing *R differ for each word, yet they all define a consistent north-to-south cline whereby *R is lost in the south. This cline, which confirms Geraghty’s observations, can be recognized all the way to southern Vanuatu and New Caledonia. Such a neat geographic distribution observed today can be interpreted in historical terms. I propose that the tendency to lose *R emerged somewhere south of Efate, at an early date in the settlement of the archipelago. This sound change triggered a range of individual lexical innovations, each of which spread across what was then a vast social and linguistic network, encompassing the whole of Vanuatu and New Caledonia. The geography of *R reflexes constitutes a fossilized picture of prehistoric social networks, as the once unitary world of Lapita settlers was beginning to break down into increasingly diversified dialects—the ancestors of modern languages.