If you do a search on Google you'll find a number of instances where employees have been sacked because of what they wrote about their employers on social media sites.
Also, many employers will check your Facebook page before offering you a job. You can often find out more about a person that way than at interview.
If you publish something (which is what you've done), whether in print or online, you are responsible for what you write. It's no defence to claim afterwards that you didn't want a particular person or company to see it.