There is no absolute threshold it is more a matter of whether or not you could argue that your decisions would have been different had you known about it.
If any ratio below is above 10%, you may consider it material
1) acquired assets / groupon total assets
2) acquiree pre tax income / groupon pre tax income
3) purchase price / total assets
Groupon has 1.7B in total assets. By looking at criteria 3, this means the purchase would have been at maximum 20% (340M - Wont get into reasons why we use 20%, but this is the ceiling) or less.
The price was likely in the 20-75M range, if living social contributed to less than 10% of the net loss of groupon (100M), and their assets were under 50M (likely).
That really depends on the situation. For instance, even a small amount might make the difference between 'profitable' or 'not profitable' and that could easily trigger sell orders and send the stock down, which would make that small amount material.
So what is material depends on the eye of the beholder.
Also, if it's good news then there will be a higher threshold for what is material than when it is bad news.
That leaves a lot of legal wiggle room.