Instagram might have copied Snapchat, Bill Gates most definitely ran into Steve Jobs on the subject of GUI, originally envisaged by Xerox corporation, and Mark Zuckerberg definitely had to settle the lawsuit with his original teammates ,who accused him of stealing their idea but all this goes to show that we cannot take implementation of an idea or a technology outside the context of its capitalistic implications (i.e how can i make the most money for myself out this idea / technology), unless the idea itself is meant for the benefit of all, in which case, it makes no sense stealing someone elses idea to start your own website, but instead its better to team up with them and make the product or technology better.
Most people who come up with brilliant sounding ideas, usually gauge its brilliance by the perception of how much money it is going to make THEM in the near term(not even the investors in the form of ROI, or the customers in the form of Operational Efficiency), and that is something as entrepreneur's one has to be careful about, especially in our saturated IT ecosystem, where everybody is vying for the same prize, and fame. Your self obsession with making money will not catch the eye of investors , who are seasoned enough to catch any form of vanity in your pitch, starting with any of the following questions
In the case of a first time entrepreneur, An idea is not worth more than the paper it is written on , unless it can be effectively executed, which mandates the idea must find traction amongst other entrepreneur's in the same industry setting. - Ergo - Please share the basic form of the idea (if required minus the USP ) with as many people in the industry as you can so that you can get good constructive feedback on the plausibility of your idea and the pitfalls associated with it.
Also If somebody is going to steal your idea, it does not imply that they are going to be able to successfully execute the idea on their own.
For arguments, sake, let us assume the person who steals your idea is an intellectual giant, and wealthy / powerful enough to create a team, build the prototype and share it in front of investors, your next doubt will be have i not lost out by sharing the idea with them. Yes most definitely , as we see in the case of the 3 examples quoted above , and that can lead to lot of angst i might assume. But one might beg to ask the question wether such a person is really in need of your idea, and if he / she is independently capable of Idea execution , they are not going to be short of ideas to implement, or friends to give them ideas. An idea is sometimes just a google search away.
Idea Execution , boils down to how interested you are in executing a particular idea, and it is at the cusp of Idea execution, that you begin to realise that you can never truly separate an idea from the entrepreneur who is deeply interested in bring the idea to life, as opposed to someone who is just interested in using the idea to make money. What happens when someone lifts an idea from another person is when the idea lands in trouble, they lose interest and move on , while the original ideator would never give up on such idea because of their deep and personal connection with the idea.
There is also the issue of Idea Chemistry , where suppose two entrepreneur's approach a great sounding idea, a certain chemistry is formed and the idea execution shapes around the common interest envisaged by the two, yet another case of a unique product creation owing to the unique personality combination of two or more entrepreneur's which cannot be replicated by anyone else in the world even if they were to steal your idea pitch, coz they have no idea about the Idea Chemistry.
Here a point that i think is important that sharing an idea with the world is not the same as sharing the working prototype of the idea or in the case of software the Requirement specification or the Design document.
Another thing i never share on any forum are sales secrets. (how to win the sale, how to get the sale, what are the keywords that customers must / like to hear in order to be convinced, how to build credibility in a particular market)
What i have learnt (and am still learning) through cold hard experience i never reveal directly , and maybe to close friends whom i trust thru' subtle hints.