Social security number for tax filing.

There is something commercial landlords might be able to do to help reduce bad social conditions when their property is in a war-zone D neighborhood.

Yes, tell them they are all on the lease and each is responsible for the full amount.You are not concerned about the social structure within the house, only that the rent is paid and the property is kept in good condition.

What kind of marketing: social media, local papers, networking meetings? 

Even ones you think may not be interested, socialize more and network (reia, meetup, any social event) so you grow the number of people you come into contact with and can potentially speak to. just dont try to SELL, youre not selling youre providing opportunities to partner on an investment. 

I know the importance of being able to be found on social media platforms and all but what are some other clever marketing strategies you all are implementing to attract passive investors?

Condos are, as one person put it, "socialized housing."

You just need to know how to use social media to your advantage.

Don't bother retaining a lawyer unless they take you to court.Just send them a letter indicating that since they defaulted on the lease they forfeit the deposit (use what ever wording applies to your state regulations).My thinking is different - spend 5 minutes on the phone with an attorney and make sure you wont be heading to court :)  Guess another reason I don't fight over small amounts (one month of rent) is that I don't want them randomly damaging the property, stopping over at my house, or complaining all over social media how I wronged them.  

Change the locks and call social services to pick her up because she is homeless.