It is only when you take into account the sum total of a person’s associations, actions, talk, relationships, you get a more complete picture of them in the real world than if you just listen to their words. The corollary in the web 2.0 world (or web 1.0 world for that matter) is that the more data points and links you have connected to a person, the more complete the picture of their identity.
There are side effects of this fact. Namely, that some peoples actions do not align with their words. Hence the old “on the internet, nobody knows you’re a dog” cartoon. But it is possible to construct a fairly trust-worthy identity by connecting yourself to your activities, friends, colleagues, and related organizations and then having some of those people endorse you. Think about your online identity as you would for a company. Both a company and you need to be found, identified as unique, associated with certain character traits or skillsets, verified as a trust-worthy entity to do business with, and so on.
Could somebody forge a fake identity on the web that is complete, convincing, and well-connected? Yes, of course. Just the way those same people do in the real world. Because your ability to forge an identity is separate from the type of identity being forged. The converse of that is: are there really amazing people with fantastic connections, amazing skills, incredible work-ethic, engaging personalities and impeccable integrity who are difficult to find either in real-life or on the web? Again, yes.
What is the final message here? Work on both parts. Be the person you want to be in the world. But then make sure that your online presence is aligned with that reality as well. Make sure that there is substance first, and visibility and style second.