Google doesn't have moderators posting messages like "uh-oh your illegal content is being taken down. Here's a list of other places to post it." Meanwhile, Reddit has precisely that. Warning: link to NSFW board, though this post isn't itself NSFW.

http://www.reddit.com/r/TheFappening/comments/2fa2a1/meta_ef...

In which the mods write: "On another note: please use other hosting sites besides imgur.com. We have a large list of whitelisted domains listed here that you should be uploading to besides imgur. Do not put all of your eggs in one basket."

The differentiation is knowledge & intent. Nobody types in random photo GUIDs after the imgur url to find content, just like nobody generates SHA-1 hashes hoping to find valid magnet links. Users follow a route of links through search engines and content aggregators to find what they're looking for. Look, I know it's easy to take this whole issue to ridiculous logical extremes[0], but the argument that you don't have culpability by hosting links is really, really weak.

To head off the probable route this discussion will take, linking to Google or Reddit as a whole does not incur the same culpability as those sites do by linking directly to the material, as finding the offending link requires additional knowledge. If you linked to Reddit along with instructions like "go to <subreddit> and click the third-highest link for the week" then it would incur culpability as it's functionally identical to linking to the content directly.

[0] http://en.wikipedia.org/wiki/Illegal_number