I haven't done this before with social, but we've done something similar. We have a page for leads on a Microsoft directory that goes straight into our Marketo system. Microsoft had all the documentation already (thank goodness!). I also know there are APIs you can use if you need to. Basically, the APIs take the information from the non-marketo form they see, populate the hidden marketo form, and then pull this back into Marketo's database with all past history. I worked with a developer to get this done: http://developers.marketo.com/ - REST APIs
There are lots of examples of posting hidden Marketo forms linked on the Community.
However none of this is going to enable a dark post to overlay a Marketo form. Facebook doesn't allow that.