So Google Analytics might be the biggest violator of HTTP. Those bastards.
"GET requests should have no side-effects" was never supposed to mean "you can't log GET requests". This timeline thing just seems to be logging parts of your web activity: a privacy can of worms to be sure, but not a HTTP violation.
That said, there are certainly very similar cases out there already. For example, when you read a thread in many forums (vBulletin, at least), it updates your public (or semi-public) profile to say you were last seen reading that thread. I've never particularly cared for that either.