In Mainland China this almost always means domestic platforms —
微信 (WeChat),
微博 (Weibo),
小红书 (Xiaohongshu/RED),
抖音 (Douyin),
快手 (Kuaishou), B
站 (Bilibili). Western platforms (Twitter/X, Facebook, Instagram) are blocked, so
社交媒体 in everyday Chinese conversation almost never refers to them.