Global COVID-19 lockdowns have disrupted adolescents' in-person social networks, increasing likelihood of loneliness. Social media can help adolescents maintain and develop peer relationships across distance. In this short longitudinal study with 735 Peruvian adolescents (ages: 11-17) from low-to-middle-income urban settings, we investigated whether online experiences relate to loneliness during initial stages of lockdown. Loneliness remained constant between week 6 and 11 of lockdown, was higher for females and similar across school-grades. Positive and negative online experiences were more frequent for older students, and females experienced more negative online experiences than males. Greater positive online experiences related to lower loneliness, with the reverse pattern for negative online experiences. Our results suggest that positive online experiences may mitigate loneliness during physical isolation.