Tell people what you do ... not how you do it ... establish a market or it isn't worth pursuing
don't feel like that ideas may steal. but not the business model,revenue model,marketing strategies. try to share your idea with the people it will help you to know about the hurdles might u going to face in future. ultimately any idea have to reach the potential customers so sharing idea is cost as advertising through mouth publicity.but u think your idea is unique and not yet developed one then think twice. facebook already having chat and photo sharing but whatsapp and instagram have their own significance .
There are a few issues you will need to address here:
1. Do you have the funding to properly protect your idea? IP work is expensive.
2. Do you have the funding to fight a court battle to protect your idea?
3. If not, will you be able to maintain your service, continually update and add new features and grow the user base?
I suspect, as for most entrepreneurs, the answers to 1 and 2 would be either "no" or "I have no idea". Which are for purposes of this discussion the same. Therefore, you need to have a robust idea, plan and feature set that you can update and maintain your position as the first and primary mover in your space.
As you see from the Snapchat v. Instagram fight, IP protection and new innovations are stolen, copied and improved upon before you could even begin to gain IP protection.
I understand your problem. Before you explain any of the details just get a standard non-disclosure and non-compete contract form basically any where on the internet. This will keep you idea confidential. Would be happy to help. There will be knock offs and competitors. Just be better. Do more. Probability is there is someone trying this or thinking about it now. Being first to market is not as vital as some think. You can learn from someone who makes mistakes. Just really concentrate on creating a good product that solves a problem. Send me an NDA if you want help.