Trying to answer whether FB advertising is effective in generating leads is similar to answering whether shipping containers are effective in transporting goods...because it very much depends on which goods you are transporting and how quickly they need to arrive at their destination.
If you are transporting basic household products, cars or furniture, shipping containers can be a cost effective method of transportation. However, if you are transporting rare collectibles, diamonds or ice cream, then perhaps it makes sense to seek other alternatives
For this reason I revert the question back to you - what FB advertising are you considering investing in, what types of leads are you looking to generate and how would you monetize these leads? Once you share these details, we will be in a better position to discuss how appropriate FB advertising is.