Apple and Facebook have yet to resolve their differences, but Apple has taken a big step in making its Ping music social network properly social by signing a deal with Twitter. From yesterday, people can link their Ping and Twitter accounts, to find friends who are on both. Once linked, whenever they Post, Like or Review a song or album on Ping, it will be auto-tweeted to their followers with iTunes download links. Interestingly, people using Twitter’s website will get a playable song preview right there without having to leave Twitter. Neither company has commented on the financial terms of the deal – presumably affiliate fees on songs bought from Twitter links is a big part of it. It certainly makes Ping more appealing, and boosts Twitter’s credentials as a music discovery service.