To share an idea online well, pick one specific claim rather than a broad topic, choose the format that suits it, and write a title that says what the idea actually is. Publish where an audience already gathers, invite criticism early, and revise in public. Clarity and a narrow scope beat polish almost every time.
Short enough that someone will read all of it — usually two to four sentences for a proposal, or under a thousand words for a first written version. Length signals confidence, but it also hides weak reasoning. If the core claim cannot survive at short length, more words will not save it.
Yes, if you share it with a small number of people who will be honest. No, if you publish it broadly. Early sharing is for finding the flaws while they are still cheap to fix; broad publication is for an idea you have already stress-tested.
Ideas are far less scarce than the willingness to execute them, so this happens less than people fear. If attribution matters to you, share it somewhere that timestamps and credits authorship publicly rather than in private messages where there is no record.