If one person knows about it they can tell anyone else who's interested. Tools like Freenet are reportedly very popular in China, since people just pass them to their friends and they're easy to use; I believe there are already Firefox extensions that allow the easy use of an alternate/extended DNS list, so those will likely be passed around in the same way.
Yes, and then what ? Freenet servers can (and, AFAIK, routinely are) be blocked by IP. Protocol analysis tools can (and again, AFAIK, already are) used to block undesirable packets regardless of IP. Perhaps more importantly, any person who is using a tool like Tor or Freenet is exposing himself to a serious risk of prosecution, incarceration, or, in extreme cases, execution. This is a risk that most people simply wouldn't be willing to take.
Referring, of course, to the proposed U.S. legislation which could cause severe damage to the Internet—at least, that's what a lot of people are saying. See, e.g., this Open Letter From Internet Engineers to the U.S. Congress (the first signatory listed is Vint Cerf). On Wikipedia, people including Jimbo Wales are discussing strategies as extreme as blanking the entire site (except for an explanatory message) to get people's attention, and thereby perhaps incite them to action, such as calling their Congressional representative.
I just happened to find out about all this a few hours ago, being someone who tries to avoid distractions like most kinds of news, so possibly others here with similar habits will appreciate having it called to ther attention. Or possibly they won't. But to those of you who possess relevant kinds of expertise:
(I think this subject can be discussed without political advocacy, in which I am mostly not at all interested anyway. It just looks like a practical problem to me.)
Edited to Add: I forgot to include a fourth bullet point:
It seems to have been assumed by many commenters, nevertheless.