I read this on 37signal's blog once - charge only for those products which enable people make more money, or save time, or be productive, or gain something in g

I read this on 37signal's blog once - charge only for those products which enable people make more money, or save time, or be productive, or gain something in general. The rest should be Free - like Facebook, Twitter, or so. Facebook and Twitter do provide value, but they dont enable me to make money or help my business per say.