If you invite the soldier into your home, you're bypassing the Constitutional protections you're granted. If Facebook gives its information to the government willingly, there is no Constitutional question to be had. CISPA was a voluntary program, you had to solicit the government in order to be involved, not the other way around.
CISPA is not me inviting police in to search my home. It's someone else coming into my home on behalf of the police, conducting a search the police couldn't legally do, and then reporting back on the results the police. And they're doing this with the encouragement of the police and with a promise of legal immunity from the police. But we're going to pretend that wasn't really a search and that the restrictions that apply to the police don't apply here.
That's not true at all, the police are entirely capable, legally speaking, of performing the proverbial search. They just don't have the manpower or the expertise.
Furthermore, you don't have to invite anyone into your home if you don't want to, and yet even further you can tell the people you ask to come into your home to not share the information they find with the police. No idea why you would do that, but you absolutely can.
Again, that makes it a bit of a non-starter, regardless of what problem it's attempting to solve.