Security! I'm not talking about general privacy and security that all SaaS vendors provide like SSL, data center security, etc. I'm referring to much more granular level of data access control which is defined and controlled at the enterprise application layer. Most companies want to control data that their users access. Take a SFA system as an example. You typically see policies that allow sales reps to see information about their own accounts and opportunities (only), sales managers to see data belonging to their teams, regional managers within their region, ... so on and so forth. So, you'd only want your users to see Enterprise "Tweets" that they should see.
Independent Enterprise Social Networking services, like Yammer, lack visibility into external system data and the data access control policies in those systems to control who should see what. Sure you can create logical groups and confine private posts to those in the group. You can even integrate in SAP, Salesforce, Oracle EBS, etc. as members of a group but you're still not going to know which specific data updates some one should have access to, at least not very easily and not without external and custom code.
Application administrators go to great lengths to set access control rules so it would be great if the Enterprise Social Networking service could enforce those same rules. Unfortunately, they really can't. First, they would need access to the rules which is not easy to come by (not their fault). Second, they'd need a standard way of defining the external application objects and access control rules which doesn't exist today (again, not their fault).
Chatter has an unfair advantage by being native to Salesforce. The same data access control rules you configure for your objects are also valid for Chatter feeds. Users can only subscribe to data that they have access to. No additional work required! However, Chatter will the have the same limitations as Yammer when it comes to data objects external to Saleforce. Salesforce does provide for the flexibility of adding custom objects and associated access control rules but you'll then need to make sure you synchronize the data with your custom object(s).
I'd say that CubeTree's sale to SuccessFactors was certainly a good move on their part. Once they're integrated into SuccessFactors, they'll be in the same position as Chatter but for HR related data objects contained within SF modules.
I hope you don't take this as a beat down on Yammer and like services but when it comes to Enterprise Social Networking, they're going to lack the necessary controls to be widely adopted.