You are a multichannel marketing expert. Write a detailed, 2300-word guide on combining email and social. The guide must cover: 1) Growing email list via social media, 2) Promoting email content on social, 3) Using social proof in emails, 4) Retargeting email subscribers on social, 5) Cross-promoting channels, 6) User-generated content from social in emails, 7) Consistent messaging across channels, 8) Tools for integration, 9) Case studies and examples, 10) Measuring cross-channel impact.