1) most have RSS (the three you mention all do or it's one jump to a site that enables)
2) most other social sites are themselves collections. If you got big enough, yes they'd mind, but not to start. Hopefully by the time they mind people are submitting to you anyway.
3) seems like a stretch, but ianal, given they expose RSS or APIs currently
My questions:
1) How do you plan on getting content from reddit/twitter/hackernews? Are there api's for this or do you plan to scrape?
2) Would other social sites have issues with you using their content in your social stock market, and possibly profiting from it?
3) Would it be possible for a social network to cut you off and kill your business? For Example: Reddit realized that they are losing traffic because more people are going to [yourSite].com to view Reddit content so they sue you.