The easy answer is no, companies are not "hacking" your Facebook account.
While it is okay to have personal social media sites, the best advice I can give is that no comments related to work be posted on personal Facebook pages.
I always recommend that both candidates and employees steer clear from postings about work or coworkers.
In this technological age, it is hard not to be exposed. Use good judgment on everything you do -- anything, from Facebook to just getting caught in someone else's pictures may have an impact on aspects of your life you didn't intend to. Be careful.
Due to the times, many are being forced to remember to lean on the professional side, even on their own private sites. If you think it could cause you issues at work, take the safe route.
Only post pictures, links, and comments on Facebook and other social media sites that you wouldn't be afraid to show a recruiter during the interview process.
I have seen a few people that have ended up "seeking opportunities" elsewhere because of comments/posts they have made on those accounts.
Regardless if a company decides to review your social media page or not, I would highly suggest making them private or increasing your privacy settings.
Most employers have a social media policy in place to guide employees on appropriate and inappropriate behavior as it relates to online posts.