This happens a lot more than we think it does, and I think we’re now starting to realize it as we witness Facebook and Snapchat copy each others’ features and incorporate them into their own products. This week, for instance, Facebook rolled out Stories, which is really just a copy of Instagram Stories (also owned by Facebook), which is just a copy of Snapchat Stories. And, yet, no one is getting sued here. Seems you can fly prettttyy close to the sun in tech and not get burned. So where is the line? Can you just slightly tweak and rebrand another company’s core product and make it your own?
The answer to this question will vary by jurisdiction. In the US:
Functionality is covered by patents. Pretend they don't exist and start building. Do NOT look for patents that cover what you do, you will be wasting money and exposing yourself to more risks..
Patents are an ugly mess right now. They cover a lot of things and, when you are successful, someone will find a way to a way to sue you. Also, if you knowingly infringed on a patent, damages are triple. In short, due diligence costs money, has no real value, and results in INCREASED risk.
To me, the bigger thing to focus on is improving over the existing competition. Usually the first mover has an advantage. How is your product or service compelling enough to overcome that advantage? Social media networks have a very strong network effect. If your product is social media, how is your product compelling enough to overcome that advantage?
I am not a layer, but...maybe, maybe not. If the company has obtained a design patent on the appearance of something or a trademark, then definitely not. So, a specific rendering of a "thumbs down" symbol *might* be off limits. If it is the idea of something, like giving a thumbs down to indicate disapproval but one that looks distinctly different, you are probably in good shape. Work-alikes, things that provide pretty much the same functionality, but present that functionality in a visual, aural, tactile, etc. package that is distinct, are likely legal. However...the more similar it looks, the more likely it is that lawyers will get involved. Can you afford that?
Then you used the term "tweak and rebrand another company's core" product. Issues:
Great things are built on the shoulders of giants.
So yes, by all means take inspiration from someone else's ideas.
The fine line is whether to make a complete clone or modify and call it your own.
The latter is more acceptable.
The hard fact is that that we cannot ring fence our ideas. Once it is out there, there is really no protection. But remember what you do unto others, others will do unto you. :P
No ideas are unique nowadays. You just need to be a bit different and better then existing competition. Facebook was not first of its kind, before it there was mySpace, but Facebook was just different and better, and it seems from your suggestion they are following that path still successfully. Those types of huge products are either too generic to sue or its not worth the money spent in courts.
Hope this helps
Rocket Internet have built a huge business copying other people's ideas. They look for other companies to copy, see the mistakes they've made and then build something similar generally faster, cheaper and better.
Lets break your question to few things .. Do you have a market? If yes, then go for it. Do you have audience? If Yes, then go for it. Are you different than others? If yes, then go for it.
Copying features is okay, but all depends on the service you provide. Users with low Internet Speed can use Instagram other features, meanwhile you need 4G+ for Snapchat to keep track with people stories. What if SnapChat introduced a new version of an app that can help people with low speed Internet? Let say, they can split the voice from each story and Introduce SnapVoice which will take one frame of the video and make a voice version as for users with low speeds.
In total, copying features is fine, but do not clone it. Give them the same idea but introduce it in unique way.