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.
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.