I've had a number of questions lately about Facebook and how would I recommend an agent "Engage" people on the site. Well, the short answer is "It Depends!" What do you want to accomplish? If you want to post your listings to other agents, then you need to have as many other real estate agents as your friends as you can. If you want to engage your LOR (List of Relationships), where most of your referrals are going to come from, then you need to have them as your friends.
Try this . . . create an A+, A, B and C list in Facebook and categorize your Friends, just like you do in your database. Then, every week, go into the A+ and A Group and look at each of their posts to see if there is anything that you can "comment" on or engage them with. Look for signs like, "My mother is moving to Seattle" - BAM - Engage and get the referral. or signs like, "We're having a baby" - BAM - larger home needer - Engage and work on the business. Remember that these are your A+ and A groups so they will more than likely refer to you anyway, as long as you care and pay attention to them.
You might also share your new listings with this group and ask if they know of anyone that would be insterested in the listing. Keep it personal and not too generic like most of the direct posts from MLS.
The other thing that I would do is post "something of value" at least every 2 weeks. Maybe a link to an article you've read about homeownership, low rates, etc... Or something that is going on in the community that you are involved with to show that you are "giving back".
Spend the time, but not tooooooooooooooooo much time. It's easy to get lost in it and then your day is shot.