New ways to use brands in social media are pressuring traditional conceptions of trademark law. Contrary to much trademark doctrine, every brand is built by a community, not by its proprietor alone. I previously described this phenomenon as consumer investment in trademarks. Internet technology amplified the effects of the consumer investment model, enabling consumers to gain more power over the marks of others. This Article shows that social media have turned the volume of consumer voices up another notch and explores the consequences for trademark law. Sites like Facebook offer consumers a platform for the expression of personal identity through trademark preferences. Social media also give consumers unprecedented power to affect brand value by publishing positive and negative commentary. If corporate brand owners want to take advantage of social media, they must let go of much of their control by opening their brands to constant consumer feedback. This trend is changing traditional notions of what it means to acquire goodwill in a mark. Brand owners no longer work alone to craft the story of a trademark. Instead, modern brand narratives are written in collaboration with consumer communities. This new trend of trademark co-authorship through social media will require rethinking some entrenched concepts of trademark law. Ironically, one way for trademark owners to reassert control of their story is by linking their brand narrative to marks belonging to others. This phenomenon occurs every time one brand owner tells its audience to “like it” on Facebook or “follow it” on Twitter. In social media, many brand owners use the marks of others for commercial benefits without express authorization. The ubiquity of this trend requires rethinking when unauthorized uses should result in trademark liability. New social media norms will require tolerance of expressive, informational and even some commercial use of marks that happen without the owner’s permission. Consequently, social media are creating multiple challenges for everyone attempting to apply trademark doctrine to new practices in cyberspace.