FB, Twitter and other social sites are under market pressures to maintain/increase user base. At the same time they must justify to skeptical customers/investor

FB, Twitter and other social sites are under market pressures to maintain/increase user base. At the same time they must justify to skeptical customers/investors that the numbers they publish are legitimate somehow. So, they must adopt a token effort to to combat this, but really it is in their best interest to allow masquerading so long as it is sophisticated enough to not be to obvious.