Yahoo today opened up a private beta-test version of its social networking application, dubbed Mash, hoping it’s broad portal reach can allow it to play catch-up with leaders MySpace and Facebook.

Mash, formerly expected to be called Mosh (a name Nokia got to market first), features a look and feel most similar to Facebook, according to early beta testers. It includes a set of modules that can be moved around a page including access to Flickr (also owned by Yahoo) photos as well as applications for blogging, guest books, friending and more. Yahoo, much like social network rival Facebook, plans to open up its application development to third-party providers. That move led to Facebook’s recent popularity surge.

Another unique feature is the ability of one user to edit another user’s pages—making Mash pages a collaborative effort among friends. Another feature, dubbed Pulse, lets users see what is happening on sites across their social network.

The big portal and search players like Yahoo, Google, Microsoft’s AOL have had mixed success with social networks. Yahoo already has a social network/blog network site, called Yahoo 360. Google’s Orkut has largely been a bust outside a few international successes. Microsoft and AOL have had success within their former ISP-driven customer bases, but those customer sets have been headed south for years.

Service providers, meanwhile, have yet to make significant investments or product roll-outs in the social network area, putting themselves at significant risk of missing this important new application area altogether.