Measure it however you want (LOC, man hours, whatnot).
Guestimates (by someone who is 'in the loop' are of course welcome)
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It depends on how you define "share". The codebase was forked I think around July/August 2009 and since then not many bug fixes and features have found their way between them. Stack Exchange still lacks hugely awesome features found in SO such as the additional search parameters.
So in lines of actual code, it's probably close. But in terms of tangible new features and bug fixes that haven't made it to one platform or the other there's quite some distance.
Most of it. :)
There are only a handful of people who can answer this (unless one of us finds it elsewhere where they already have), but you can get a sense of it from listening to the podcasts.
However, I can't see how an exact figure would helkp—LOC and man hour measurements are known to be problematic in the first place. I'm sure it's not secret, but it would take them some effort to give you a reasonably accurate answer (at least more accurate than mine above), and I'd much rather they spend that on more relevant requests.