Facebook has its own logging system - http://en.wikipedia.org/wiki/Scribe_(log_server) - which they probably use for in-house systems. I would guess that the library in question might be used when they run third-party applications.
I am afraid, legal help will cost us much more than our limited user base would be able to pay. That's why we don't actually want to register our own company, but rather are looking for a company that provides service similar to what I described.
It's hard to believe that in 2013, when you can do anything online, nobody came up with such a simple service of handing payments and paying taxes for others.
Thanks a lot for the reply. Unfortunately, we are not based in US, and registering a company is much more complicated in the country we are living in. And, as I said previously, we would like to keep focused on coding, and pass all the legal details to somebody more competent.