links between two users has been shown to be a good predictor of friendship in online networks [@adamic2001]. Figure \[fig:structure\] shows the Adamic/Adar metric of neighbourhood similarity across the various single and multilayer neighbourhoods described in Section 3, and the four link types.

The Adamic/Adar metric is distinctly higher for multiplex links. In agreement with previous studies of tie strength [@Gilbert2009], we observe that multiplex links share a greater overlap in all single and multilayer neighbourhoods. In single-layer neighbourhoods (Figure \[fig:aat\] and \[fig:aaf\]) we observe that after multiplex links, those links internal to the network under consideration have a higher overlap than exogenous ones (*to* in Figure \[fig:aat\] and *fo* in Figure \[fig:aaf\]), followed by unconnected pairs, which have the least overlap.

With respect to the multilayer neighbourhoods, we can observe a much more pronounced overlap across the link types. While the global neighbourhood overlap follows a similar distribution to the single-layer neighbourhoods but at a much lower scale, in Figure \[fig:aai\] we can observe more clearly that unconnected pairs share little if any neighbours, while multiplex links have a significant overlap. With respect to the global neighbourhood (Figure \[fig:aau\]), both Foursquare only and Twitter only links share significantly more 