The best thing is it's free! Even better is that if you pick the right groups you can target the people you want to target. Many groups are set up for a reason, be that to sell things in an are or promote business in an area, maybe even to promote an interest to a group of like minded people.
So why not pick a group and start posting? No point falling foul of the posting police, so check out the rules of the group (many are set up by people who like to say say no to this that and the other) but many allow you to promote your business especially if you can do so without actually selling too obviously.
You might want to promote your business to other businesses locally and there are many groups for that or you might think a larger audience might be nice. Just search for groups, check out the rules, join and post away. Suddenly you are reaching far more people than you used to.