Sometimes one just happens to hit the Zeitgeist. Of those 439 shares, only 33 (as far as I can tell) are people who are actually friends of mine on Facebook. That means that on average each of those 33 shares gave rise to another dozen or so. No doubt there will be many others who, like me, saved the image locally and reposted it.
It's a bit embarrassing because I have no idea where the original image came from, unless the original tweeter made it herself (and it doesn't really seem like her style). So my profound and sincere apologies and gratitude to the artist, if he or she should ever read this.
Lessons from this for attempting a viral tweet or Facebook post:
1) Strong visuals are key. The Tom Baker story is just a block of text, but it tells a very moving tale in a few words. The picture of the popes has no text at all, but hangs on a very few very recognisable images - the popes themselves, the flags.
2) Timing is everything. The Tom Baker post came just a week before the Whonivesary, at a time when attention on Doctor Who was possibly the greatest it has ever been, and Tom Baker's unexpected return to the screen boosted the post even more. And you can't do much better than post a joke about the World Cup final in the first few seconds after it has been decided who is playing in it.
3) It's very difficult to plan ahead. Both of these were whims posted just before I went to sleep. I have put a lot of stuff online, and most of it is my own content rather than page-grabs from magazines of other people's memes. Very little of it gets this sort of attention. I deliberately followed both the above rules with my posts about Sarajevo three weeks ago; but with much more modest effect. Sometimes it's just a matter of good luck; though of course it can also be true that the harder you try, the luckier you get.