Stateside, there are two frequencies available for fixed-wing air-to-air use and a third frequency if you are flying rotary wing.
122.75, 122.85: Fixed wing/private airports
123.025: Rotary wing
(from our domestic AIP: the "AIM," paragraph 4-1-11)
Often hear pilots flying formation "X flight, go 123.45," even though that frequency is not approved domestically for that use. I hardly ever hear chatter on the approved air-to-air frequency.
I suppose that when the oceanic area controls agreed upon 123.45 for the Caribbean, North Atlantic... that it brushed onto pilots as the common air-to-air frequency. Hence it is simple enough to remember.
As for you guys who fly across FIR boundaries every 15 minutes on a short sector, the frequency for air-to-air is probably buried deep inside multiple AIPs.
With 8.33 spacing, there ought to be a frequency like 136.992 that is not being used somewhere within an entire continent.