It's fun that we consider 3 minute clips to be "long form" nowadays. When YouTube was being bought people said that it only consisted of short amateur clips and copyright violations; two things advertisers would supposedly not be interested in.
Now I'm not saying that Vine is the next YouTube or even that it is monetizable, just that "we haven't figured it out yet" is not a convincing argument, and surely its userbase would be appealing to some buyer.