社媒 is a clipped form of 社交媒体 (shèjiāo méitǐ) 'social media,' common in marketing and industry talk. In casual conversation, people more often name the specific platform (微博, 小红书, 抖音) rather than say 社媒.
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Industry/professional shorthand. You'll see it in job titles like 社媒运营 (social media operator) or 社媒投放 (social media ad placement).
