Communication has long been concerned with people’s media repertoires, yet little of this approach has extended to the combination of social media platforms that people use. Despite their considerable popularity, research has found that people do not select into the use of social network sites (SNSs) randomly, which has implications for both whose voices are represented on them and where messaging can reach diverse people. While prior work has considered self-selection into one SNS, in this article we ask: how are different SNSs linked by user base? Using national survey data about 1,512 US adults’ social media uses, we build networks between SNSs that connect SNS pairs by user base. We examine patterns by subgroups of users along the lines of age, gender, education, and Internet skills finding considerable variation in SNS associations by these variables. This has implications for big data analyses that depend on data from particular social media platforms. It also offers helpful lessons for how to reach different population segments when trying to communicate to diverse audiences.