And here I thought social networking sites were already for dating. Zoosk, an SF-based startup, has raised a $4.1 million second round for a dating service that works atop social networking sites, like Facebook, Bebo and MySpace. The round was led by Canaan Partners, with participation from past backer ATA ventures. The company’s earlier round was not specified. The company says it improves the online dating experience by helping people “meet and connect through the context of their online social communities.” Again, that seems like what social networking sites themselves are for. An aspect of the service is that it can connect users on different social networking sites, so that is unique. The service claims 400,000 daily unique visitors. Release.