Have you told Groupon about it yet?
If the sign-up-before-use is not listed under the terms and conditions when you bought the coupon then you should be entitled to your coupon.

Now I'm scared. Cause I bought some facial treatment from MilkADeal and haven't use it yet. Later the shop same thing like this.

That's bad. Can people just boycott them once and for all?
I mean service business, you are suppose to treat your customers nicely. Who will want to go there if they know they will get insulted. One of the main factors is the workers' personality.

I don't think it's Groupon's fault. It's only certain deals like this one or something similar like Facials and Skin Care stuff. Similar businesses like Bizzybody are the one that we should be careful with.

Yes, there are Groupon deals for Malaysia and there are so many other group deals in Malaysia.
http://forum.lowyat.net/topic/1748708

I would say it's safe. It isn't GROUPON's fault for having Bizzybody to do what they did.
I've purchased a couple of coupons from many group buying websites, and so far have never encounter this sort of problem yet.

Holiday packages I'm not sure but make sure you read the T&Cs properly and know what your rights are as a consumer.
I've used coupons for mani/pedi sessions, facials, spas, pet grooming and makan-makan so far everything is good and I'm very satisfied with it.