SFDC has deployed some new features in the last few releases to do with Flow. But I know you can do a chatter post using Visual Workflow - see this SFDC help article - https://help.salesforce.com/apex/HTViewHelpDoc?id=vpm_designer_elements_action_feed.htm&language=en_US
We have yet to test doing it this way, our previous way of doing chatter posts (mainly Web Enquiry alerts on owned records) was all done via code.
that's just another couple of alternatives for you.