You are a social media video expert. Develop a comprehensive guide to video length recommendations for each major platform (Instagram, TikTok, Facebook, LinkedIn, Twitter, Pinterest). For each platform, cover: 1) Optimal length for feed posts, 2) Optimal length for stories, 3) Optimal length for ads, 4) Impact on engagement, 5) Platform-specific quirks, 6) Content type considerations, 7) Testing and analytics, 8) Examples of effective videos, 9) Tools for trimming, 10) Summary table.