This paper explores how online social networks and co-presence social networks complement each other to form global, fused social relations. We collected Bluetooth-based co-presence data from mobile phones and Facebook social data from a shared set of 104 students. For improved analysis accuracy, we created weighted social graphs based on meeting frequency and duration for co-presence data, and based on wall writing and photo tagging for Facebook data. By analyzing the overall structural properties, we show the two networks represent two different levels of social engagement which complement each other. By fusing them together, the average path length and network diameter is shortened, and consequently the social connectivity increases significantly. By quantifying the contribution of each social network to the fused network in terms of node degree, edge weight, and community overlap, we discovered that the co-presence network improves social connectivity, while the online network brings greater cohesiveness to social communities.