> Google can't index Facebook content or Twitter content even though it's public
They index plenty of their content [1][2]. What they don't index is content not explicitly marked as "Public". e.g. Facebook posts with visibility settings or protected tweets. FB, Twitter, and now LinkedIn have plenty of content that's not publicly accessible: that is, content which doesn't require a logged-in account that explicitly agreed to TOS/EULA, but they have tons of publicly-accessible content, too. The latter is fair-game.
That said, the Craigslist lawsuit is still bewildering to me. That content is explicitly public, does not require a login, and the only agreements are the automatic unforceable browsewrap ones. The LinkedIn v. HiQ case is very similar to the Craigslist v. 3Taps, however the decisions are in opposite directions.