There has always been repercussions for socially unacceptable speech. What the internet has affected is who can hear or read your speech.
If I go into a supermarket and shout, "I hate black people and they shouldn't even be in America!" I am going to suffer the consequences, but probably only from the people who heard me. Everyone else will have heard it second-hand and may due to social circle pressure decide to sever ties with me. The only way I'll actually suffer long-term (other than being beat up by a mob) is if I was famous since people like it when famous people do things.
The problem is, people continue to think that "The Internet" is just some kind of throwaway void where they can spout whatever they want. Justine Sacco probably isn't a racist, and was probably making a joke about "white privilege" or "white bubble". But she showed poor judgement in assuming that writing that in a public space would be benign. Even with her lack of followers. Text is not a good way to facilitate a joke that requires context. Her joke about AIDS and Africa and white people not being affected is based on some context no one but herself had.
If _I_ am at the supermarket and I hear some old cranky dude yell that out. It's a story I'll probably tell my friends about. They in turn may decide to use that as an inside joke/catchphrase of our circle of friends. But there is no way I'll repeat that to someone else outside of the group of friends that know what the context is. I definitely won't post it on Twitter, not without at least having posted the story about the crazy old man in the supermarket.
So I kinda laugh when people say they can't say anything that's not "PC" anymore. That to me just means they want to really spout some really offensive opinions to a public audience without paying the consequences of doing so. Then of course when there is kickback, they blame it on people being too "PC"