I have set the plugin preference to enable publishing. I have also enabled period tasks. When I run the periodic task, the action stream updates internally but does not necessarily publish. It sometimes publishes and sometimes not.
I'm on 4.3 but had this problem with 4.261 also. Upgrading to 4.3 made publishing happen one time when I ran periodic tasks. I'm wondering if it has something to do with LaunchBackgroundTasks which I had set to 1.
Reported on Movable Type 4.2
As it turns out, it seems action streams first collect your actions and then put them in a publishing queue. So publishing is not instantaneous, but it does happen eventually. I've got a cron job running every 5 minutes to run periodic tasks. That gets all the action streams collected and then later, eventually published.