If you're interested in tracing the origin of the quote, you might submit it at fakebuddhaquotes.com. Bodhipaksa does an amazing job there of tracking down the actual sources for the many mangled, made-up, or misattributed so-called "Buddha quotes." Occasionally, he will also find that one of the suspicious-sounding quotes has an authentic Buddhist source.
Bodhipaksa's response to my email:
"The Buddha used the "earth versus what's under my fingernail" simile dozens of times.
He used it, for example, to compare the large amount of suffering destroyed by one who has entered the stream compared to the amount of suffering remaining, and to contrast the large number of beings not born human compared to those who are born human.
The closest I've found, though, is in the Samyutta Nikaya, page 1879 in Bhikkhu Bodhi's translation. After the introduction of the simile, we're told:
"So too, bhikkhus, those beings are few who possess the noble eye of wisdom. But those beings are more numerous who are immersed in ignorance..."
But this is rather different from your quote, which is not about how many people have attained awakening, but is about how many will attain awakening. And your "suspicious poster" is, I'm sure, right in thinking that that version of the quote would put people off seeking awakening.
So I'm fairly certain that your quote is actually a misquote.