You mean clones of existing business models? If this path wasn't profitable, Burger King/McDonalds/Jack-in-the-Box couldn't co-exist. I've been really confused by this pervasive idea that seems to be going around that the only way to make money, is to think of something no one has thought of before. If that was true, Google/Starbucks/KrispyKreme/Facebook wouldn't exist. People that create new markets tend to make far less, than people that expand/perfect existing ones.
I definitely believe that clones could have a profitable investment, but the hardest part is actually making it come true. Most people don't embrace change too willingly. "Oh another facebook?" even if you mention the small extra features that you have that the original site doesnt.
If you have a good sized number of people who could genuinely use your product (better if its a specific niche), then it would really help. I'd say do thorough research, ask friends and family, and definitely ask a few not so close friends as well for input (close friends and family will try your idea just because they're close to you, not always the best advice).
None of the companies you have mentioned have reached any sort of critical mass. Clones in those environments probably have some sort of edge that allows them to gain traction in market segments the biggest companies can't touch.
Take Groupon. For relatively low-margin services, the vig is too high to use it (an OPEN forum discussion suggested that in some circumstances you may end up receiving 10% of retail, so you should either have 90+% margin or hope that yo get repeat customers), and that leaves an opening for companies with a smaller vig (e.g. LivingSocial)
The clone can be profitable if you are able to make it appealing enough to a specific niche, because you can then have hyper targeted content and ads. I wouldn't think an exact clone of Quora or Groupon would work, but a version tailored to a specific Groupon niche would be able to have deals more targeted than a general one, and a Quora clone could have ads that appeal to the questions on the pages that are niche-specific. Also, look for ways to mutate and twist the existing version, not just simply clone it completely.
That sounds way too absolute. The point is "can you add value?". This doesn't require you to be "better" all across the board, you just got to have a reason why people would choose you instead of your rival. Lots of businesses are basically only different from each other due to the branding (and often the target demographic).
