Social media, for many of us, is helpful precisely because the companies we work for don't control it. We can grow an independent audience—one we can take with us to other jobs. Setting up on a company instance would mean giving control of that to the companies we work for. Now, granted, the companies *could* allow us to take our followers with us when we leave. I doubt they would, though, which would mean needing to start over building a network every time.