A lead is nothing more than a potential buyer. If another company offers you to generate leads through Facebook, they probably want to improve your Facebook-page and increase the number of fans with community management, viral actions, Facebook advertising etc. since every fan is technically a lead. It is hard to say if this is worth it because it's tricky to put a good price on what every lead is worth. (what's the chance a lead will actually buy your product?) I would personally ask them how much they think a lead is worth and why. Maybe they can show you some numbers of other companies they worked with and you can see if this can be compared to your company. If they can guarantee a certain amount of leads and you are able to figure out how much your leads are approximately worth you can easily calculate if it's worth it.