Video sharing is an increasingly popular application in OSNs. The client/server architecture deployed by current video sharing systems in OSNs costs a large amount of resources (i.e. money, server storage) for the service provider and lacks scalability. The privacy constraints in OSNs, the current peer-assisted Video-on-Demand (VoD) techniques are suboptimal if not entirely applicable to the video sharing in OSNs. To reduce the video startup latency, we propose a pushbased video prefetching mechanism in SocialTube. In SocialTube, when a source node uploads a new video to the server, it also pushes the prefix (i.e. first chunk) of the video to its followers and to the interest-clusterpeers in the interest clusters matching the content of the video. The prefix receivers store the prefix in their cache. Those interest-cluster-peers and followers who are not online when the source node pushes the prefix will automatically receive it from the source node or the server once they come online. After the source node leaves, the responsibility to push the prefix falls to the server. Since these followers and interest-cluster-peers are very likely to watch the video, the cached prefixes have a high probability of being used.