The "friendship paradox" (Feld1991) refers to the fact that, on average, people have strictly fewer friends than their friends have. I show that this over-sampling of the most popular people amplifies behaviors that involve complementarities. People with more friends experience greater interactive effects and hence engage more in socially influenced activities. Given the friendship paradox, people then perceive more engagement when sampling their friends than exists in the overall population. Given the complementarities, this feeds back to amplify average engagement. In addition, people with the greatest innate benefits from a behavior also tend to be the ones who choose to interact the most, leading to further feedback and amplification. These results are consistent with studies finding that people consistently overestimate peer consumption of alcohol, cigarettes, and drugs; and, can help explain problems with adolescent abuse of drugs and binge-drinking, as well as other behaviors. I also discuss how these results change in cases of strategic substitutes, where individuals overestimate free-riding by peers.