This behaviour was common in the last Facebook Hacker Cup too. Most of the announcements of rounds starting included discussion of (and in some cases links to) solutions.
I guess it's part and parcel - I think Facebook did take action against obvious plagiarisers. If not, the later rounds will weed out the cheaters in any case.
I think you are missing the point here , instead of 3 cheaters three honest coders could have qualified to the next round , which would in turn give them the chance for competing in round 2 and round 3.
If someone barely missed passing through 1st round, chance for him to make it through any of the following rounds is practically non-existant. Exception being if he's experienced competitor who fell short for some reason.
Well, it will only be the last 3 spots that are (substantially) affected by 3 cheaters - everybody else either qualified or failed to qualify anyway.
And the luck I'm referring to is in timing. If you look at the scoreboard for round 1B, 1000th place scored 34 points and a time of 1:30:38. 1001st place 34 points and a time of 1:30:48.
Imagine this scenario ,It is round 1C . A talented programmer started competing there are only 20 minutes to go. He solves the Small and Large test cases for Problem A ,and only the small for problem C. But Alas ! his rank is 1002 , he doesn't qualify even though on topcoder he is in the top 200's.
I guess it's part and parcel - I think Facebook did take action against obvious plagiarisers. If not, the later rounds will weed out the cheaters in any case.