Depends on the country. In the UK I'd probably go to the Law Society website and search for a B2B lawyer under "Business -> Dispute Resolution" or "Media IT and intellectual property" (https://solicitors.lawsociety.org.uk/). Having used lawyers like this at start up companies in the past they are not cheap. However in the first instance you'd probably just want them to send a letter on headed notepaper, which might be enough to get someone's attention at Facebook and get the matter resolved.