Well you can always have them sign an NDA. That might help in some situations. Talk to an attorney - they might charge you $300 - $1,000 for their time and advice, but then you have a relationship that will be more valuable in the long run.
To some extent it will be very difficult to protect from clones. The best ideas are the ones that are carefully orchestrated, and which develop a large user base. Obviously, you need to find some people you can trust.
Your idea is probably not as great or unique as you think. Worry about getting it built rather than getting it stolen.
The facebook example is great--the general concept of social networking was around for decades (see good old fashioned BBS services of bygone eras). Facebook won on execution and strategy, not on any particular technical merit.
I don't think that IP can be protected to easily. If you take a look at many of the Browser-Based MMORPGs out there, they have very similar features. If IP were easily enforceable, then i guess a lot of them would not be around.