Facebook is actually the best example of why not to share. Remember that lawsuit Facebook went through because it turned out that Mark Zuckwhatever had actually lifted the idea from some fellow students at Harvard? Facebook bought out the company at well above market value as a settlement.
As the owner of a small tech business, I would suggest that you consider using NDA's and MNDA's whenever possible. It allows you to share, but people with money and ideas of their own will know that you are both serious and collaborative. I've been told by other companies and individuals that they knew we were serious when we showed up with NDAs or MNDAs.
And to echo an earlier comment, no the value of ideas is what drives most of our economy these days, and let not forget our favorite slashdot subject of patent trolls. You should cover your ass legally if you think you really do have a good idea. Non-Disclosure agreements and Mutual Non-Disclosure agreement are available online for free to customize.
(for instance)
Bit Law Sample NDA
Don't trust anyone who JUST tells you your idea is worthless. They either don't recognize the value, are an ass-hat, or a thief. If your idea really is worthless, someone who really understands your idea will explain it in a way that will totally convince you that it is either dumb or overdone or impractical. Otherwise, you should be signing NDA's for everything and ignoring the dweebs here that say it doesn't matter. They don't know what they're talking about. If you told me your idea and I didn't sign a contract I'd take it if I were a jerk. Shit, I've even got a co-location, a couple extra servers, a perl/javascript developer and a web designer not to mention a couple extra quadcore rackmount servers.
So now that I think about, yeah.... NDAs and MNDAs are the dumbest thing ever. Your idea is worthless and stupid, and I'd be happy to hear all about it under no legal obligations with your full technical disclosure...