That depends on your definition of a "plain old graph". If you allow arbitrary numbers of edges between nodes and allow edges to carry data, everything will be ok, right? (...right?)
You're ignoring the primary (marketed) purpose of these extensions - social sharing. The main value-add is that users can easily +1 (share, tweet, whatever) from whatever page they're on. A secondary feature of some (including this one) is to show how many other people +1'd (etc) it. There are other extensions that exist with just the first part and not the second (though they're usually not 'official') and I imagine those wouldn't be "tracking" your activity (except what you're sharing). But, tracking isn't really the main point anyway. Facebook and Google can do enough tracking without browser extensions simply through market saturation. As far as I see it, this extension just provides features that some users want.
Additionally, these extensions take up minimal space in Chrome. The screen real estate each one gets is the same size as the settings (wrench) icon. Sure, you could fill up your browser UI space with them, but it's much more difficult than it is with IE and there are more hoops to jump through in order to get them there. IE users were (are?) plagued by toolbars because they can be installed externally from the application. As far as I know, Chrome extensions can only be installed from within the application after several prompts and confirmations.
I think it's more likely that the cause is the initial high-engagement period that budding startups have. The infographic shows the average workday being 9am-12am. when you're working 15 hour days you don't really have time for anything else, regardless of what town you're in.