For social enterprise solutions (e.g. Yammer), which company is the current market leader now?

First of all Yammer isn't a "Social Enterprise" solution. Salesforce were rejected from using this term which is used around the world for mission-driven businesses http://techcrunch.com/2012/09/04/salesforce-bows-to-peer-pressure-withdraws-social-enterprise-trademark-application/ On software for organisations who want to keep threads of conversationand discussion happening, you could use a variety. I would highly suggest: http://loomio.org and http://slack.com Yammer is a big player but the product has gone massively down hill. We've tried all sorts of others, but Slack & Loomio works a treat.

My perception is that Jive Software is probably the leader; or at least a leader.  The Gartner Magic Quadrant stuff has them in a pretty strong leadership position, last I saw. Disclaimer:  I'm working on a startup that will compete in the ESN / SBS / SKM space, so I may not be completely unbiased.  But I don't think this particular answer is particularly controversial.  After all, we don't care who the current leader is, we're gonna knock 'em off their perch anyway.

We have been using Flock (http://flock.co) since some time now and it has completely replaced email, and other sluggish workplace communication platforms for us. It works on all devices and you can use it for unlimited number of users (best part: new members are automatically added, and you can invite clients too). It also lets you create groups and share files & images which makes work and decision making super-quick. Have a quick look at the website - http://www.flock.co  Check out the brief video here- Like Srinivas said in his answer, it doesn't matter which one's the market leader. What matters it how it solves yours & your team's requirements. Flock has made workplace communication quick and easy for us.

An observation.  These type of tools succeeded in the early days because of being so simple its blindingly obvious how to use them.  As time has gone by the vendors feel the need to increasingly compete on functionality and sometimes that complexity create barriers to use. There is a danger of winning the argument but losing the war.  Just a thought !

How important is that? They key is if they satisfy the use an Enterprise is looking for and it fits your pocket. There are many more like Yammer and Jive out there in the market. Disclaimer: I work in a company that has a product that competes in this space

Teams communicate across department, geography and application.  Thus, it's important to consider a social platform that is application/vendor agnostic and will equally support whatever applications you choose to use.