This document discusses how co-location information shared on social networks can threaten users' location privacy by enabling more accurate localization of users' locations over time. It formalizes the problem of quantifying privacy risks from co-location data and location information, and proposes optimal and approximate localization attack algorithms to incorporate co-location data. Experimental evaluations on mobility trace data show that considering a single friend's co-locations can decrease a user's median location privacy by up to 62%. Differential privacy perspectives are also discussed. The study aims to quantify the effect of co-location information on location privacy risks.