Analisis strategi konten promosi portal pembelajaran daring di media sosial di masa pandemi (studi pada Ruangguru, Quipper School, Rumah Belajar, dan Kelas Pintar) = Online learning platform promotional content strategy analysis in social media on the Pandemic Period (Case Study on Ruangguru, Quipper School, Rumah Belajar, and Kelas Pintar)