Most social media channels are lossy where videos are transcoded to reduce transmission bandwidth or storage space, such as social networking sites and video sharing platforms. Video transcoding makes most video steganographic schemes unusable for hidden communication based on social media. This paper proposes robust video steganography against video transcoding to construct reliable hidden communication on social media channels. A new strategy based on principal component analysis is provided to select robust embedding regions. Besides, side information is generated to label these selected regions. Side information compression is designed to reduce the transmission bandwidth cost. Then, one luminance component and one chrominance component are joined to embed secret messages and side information, keeping the embedding and extraction positions in sync. Video preprocessing is conducted to improve the applicability of our proposed method to various video transcoding mechanisms. Experimental results have shown that our proposed method provides strong robustness against video transcoding and achieves satisfactory security performance against steganalysis. The bit error rate of our method is lower than state-of-the-art robust video steganographic methods. It is a robust and secure method to realize reliable hidden communication over social media channels, such as YouTube and Vimeo.