Apart from their obligations under the law, AOL did not police content. In one example, AOL banned explicit discussions of homosexual activity but did not ban anti-homosexual hate speech. They policed the first type of speech because the CDA required them to prevent the transmission of explicit material to minors, but they did not police the second type of speech because it was not illegal.
Facebook and Twitter are actively policing speech that is not illegal.
There have always been alternatives to the walled gardens, they have never been larger than their walled alternatives.