You are correct, oetb!
I just revised #6 on that old post as follows:
“6. The Buddha said, “..bhikkhus, sentient beings reborn as humans are few as this bit of sand on my fingernail. But those not reborn as humans are many as the sand on this great Earth. Therefore, you should strive diligently and without delay to end this suffering in the rebirth process”. That statement appears to most people as an exaggeration. That quote is from the post, “How the Buddha Described the Chance of Rebirth in the Human Realm”.”
Regarding the second part of your above comment:
Yes. It is extremely difficult to “come back up” once born in an apaya.
As for humans, the problem is the following. Many people engage in good deeds. However, it is very easy for a human (one who has not grasped the anicca, dukkha, anatta nature) to be TEMPTED by sense desires.
– By the way, the fraction of HUMANS getting a rebirth in a good realm (after the human bhava, which may last thousands of years as Lvalio pointed out above), is higher than what the Buddha quoted in the above sutta.
– Those in the apayas are MUCH MORE numerous than the human population. There are more living beings (belonging to the animal realm) in your backyard soil than the entire human population on Earth! That is why the “sand on a fingernail” analogy applies to ALL living beings.
– If we just take the humans who are reborn, they have better odds of being born human again. Still, it is quite low.