I don’t know about you but accepting a friend’s post on Facebook for others to hire you for design/development work as compensation, yeah, that won’t pay your bills. Especially since they are not obligated to do so. You’ll be waiting for quite some time, probably in a box out on the street if you go about it that way. Charity work is a different story. You’re doing work for a cause.
Where are you getting this from? There are loads of professionals who do this sort of thing to get started. It’s just not in web design.
The points Alen made a valid in my view — set boundaries and let them know why you are doing it. Starting out with a few free projects is a great way to start building relationships and trust. Of course, you’re going to have to know when someone is taking advantage of you, and that part isn’t really hard.
There aren’t many costs involved either, other than your time. I’m sure a guy in a box down the street would love to start a business by doing things for free to get out of that box, eh?
Anyhow, not everyone sees things the same. However, I think belief also plays a major role in success. So do what you believe you can do.