The percentage of people who pass on a message or file.
Pass-along rates are a measure of word-of-mouth marketing. Objects typically passed include email messages, Web pages and multimedia files. Content typically passed includes humor and entertainment, late-breaking news, shopping specials, and technical gizmos.
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How-To Do Viral Marketing: The Ultimate Archive of Videos & Podcasts
MarketingTerms.com (February 15, 2012)