I'm suddenly fascinated by this phenomenon that until 2 minutes ago I had no idea existed. Presumably there are similar groups for each month and year combination over the past couple of decades or so? I wonder if the numbers and demographics are broadly the same in each group, or if there are significant differences.
Also, statistically, they reckon there's a 1/23 chance that any two people in a large enough group will share a birthday. So the odds presumably decrease if we're just talking about months, not days, but then increase again as you add years. Again, I'd reckon about 20 years, given the length of time people have been online. So, say, 240 groups? I don't know maths well enough to calculate the probability of any two people's children sharing a birth month and year, but it's got to be somewhat higher than day and month, right?
The kicker, of course, is how many of those people choose to join the relevant appropriate Facebook group.