Competition already exists. Almost every major enterprise software company minus Oracle. Microsoft does have Yammer, but IBM has Connections, Salesforce has Chatter, TIBCO has Tibbr. There is an independent, Jive. There are also upstarts like Slack.

Facebook would be entering a market that is already crowded. I'm not saying they couldn't make a big splash, only that it's not monopolized by Yammer.

If you give credence to IDC, IBM is actually the market share leader, with Jive in the second position and Microsoft in 3rd.