of common hashtags:** Similarity between users on Twitter can be captured through common interests. Topics are commonly expressed on Twitter with hashtags using the \# symbol. Similar individuals have been shown to have a greater likelihood of forming a tie through the principles of homophily [@mcpherson2001].\
**Number of colocations:** The number of times two users have checked-in to the same venue within a given time window. In order to reduce false positives, we consider a shorter time window of 1 hour only. Two users at the same place, at the same time on multiple occasions, increases the likelihood of them knowing each other (and having a link on social media). We weight each colocation on the popularity of a place in terms of the total user visits, to reduce the probability that colocation is by chance at a large hub venue such an airport or train station.\
**Distance:** Human mobility and distance play an important role in the formation of links, both online and offline, and have been shown to be highly indicative of social ties and useful for link prediction [@Wang2011]. We calculate the distance between the geographic coordinates of two users’ most frequent check-in locations as the Haversine 