overlap (scale is higher on x axis) than when observing the single-layer neighbourhoods in Figures \[fig:aat\] and \[fig:aaf\]. This indicates that some common neighbours lie across layers, and not just within, *the global neighbourhood revealing a more complete image of connectivity, which stretches beyond the single network*.

The core neighbourhood overlap is most prominent for multiplex links (Figure \[fig:aai\]), which indicates that they share more friends across networks than any other type of link. While this is expected, it *confirms that the neighbourhood overlap is a good indicator of multiplexity in ties*, and is particularly strengthened in its weighted form through the Adamic/Adar metric of neighbourhood similarity.

Multiplexity and Interaction
----------------------------

The volume of interactions between users is often used as a measure of tie strength [@onnela2007]. In this section we compare how the volume of interactions reflects on multiplex and single-layer links. We consider the following interactions on Twitter and Foursquare:\
**Number of mentions:** This interaction feature simply measures the number of times user $i$ has mentioned user $j$ on Twitter during the period. Any user on Twitter can mention any other user and need not have a directed or undirected link to the user he is mentioning.\
**Number 