vineTwitter launched a brand new iPhone app last night called Vine, which helps people share six-second looping video clips with sound to the social network, as well as to Facebook.

Based on a video-sharing startup acquired by Twitter last year, Vine’s makers say the clips are “little windows into the people, settings, ideas and objects that make up your life. They’re quirky, and we think that’s part of what makes them so special”.

Twitter co-founder Jack Dorsey went further: “Vine is the most exciting thing I’ve seen in a while. Not just because of the team, because it brings an entirely new art form to the world.” An entirely new art form if you’ve never seen an animated GIF, obviously.

Support for Facebook was a welcome feature given the recent friction between Twitter and Facebook, but the bonhomie didn’t last long. Facebook already appears to have blocked the Vine app from accessing people’s Facebook friend lists.

It’s truly disheartening, and part of a pattern that’s seeing large social networks act more from anti-competitive reasons than from what they think might actually benefit users.

Facebook seems caught between being a walled-garden social network fending off rivals, and the social-graph connected backbone for other services that it claims to be.