If I recall the argument against Facebook here, the point is: let's assume that you found a company, file for a patent and implement it using some open source tools/libraries, also backed by Facebook. Now somewhere down the line FB decides that your product is really great and they can make something similar work for their platform, so they pour resources into developing their version of your solution. You are pissed, and take them to court - but you yourself are now in breach of licence, so before you can take them to court, you need to invest (possibly a lot of) time and resources to move away from those libraries - and in the meantime their competetive product with their whole financial baking is probably giving your little one a pounding which you may not survive.

If you hadn't had used any FB libraries in the first place and Facebook decided to copy your product, you would at least have a chance to make your case in court in a timely manner.

But I'm also from Europe and may not understand all the intricacies of US justice system, so feel free to correct me if I'm wrong.