Facebook groups can be very helpful to find leads. You can join those groups and check out the members. You can also find their companies and websites.
Let’s give an example: You are looking for leads in law sector. You also need to find lawyers and their firms. Basically, you type the word you looking for and search in the group section.
As you can see, there are lots of group that have lots of members. These groups are not public, so you need to send a request for joining and answering the questions that group owners prepared. Just answer the questions properly and join the groups.