Book I mailed in Feb that PO tracking says was delivered on 28 FEB, is now listed by system as 'lost in mail' despite my attempts to follow up.  On 7 March I PM'd the requestor who has yet to respond, then last week I sent all the info to PBS through the 'contact us' system. Their reply was to let them know once the book went lost.  I did that today and am awaiting action.  

  How long do you think we should be waiting before addressing such an issue? Now that we have free tracking on our media mail packages, should we still wait over 3 weeks before expecting action, especially if a member involved in the transaction is unresponsive?

According to the Help Center:

If you sent the book with tracking purchased elsewhere (not from PaperBackSwap) and it has been recorded as delivered by USPS but hasn't been marked received yet:

• First, contact the requestor from the active transaction (in the Books I've Mailed tab in My Account), or from the "lost" transaction in your Transaction Archive if the book has been declared Lost in the Mail by PBS.
• If the requestor does not respond to your PM within a few days, contact us and we will look into it.
• We cannot grant Book Credit for tracking or postage purchased outside PBS, but we can check if this requestor has become inactive.
• If this requestor is inactive, your book should be marked received by the system.

I can tell you that if a requestor hasn't marked a book received 4-7 days after it's been scanned received, I generally send a polite PM with the tracking information, basically saying that according to the tracking info, this was delivered 5 days ago. If you have indeed received it, can you please mark it received so that I can get my credit for mailing it. If there's no response to my PM 1 week later, I forward the information on to TPTB as per the HC guidance above. Every time they tell me that if the member is inactive that the system will mark it received for me, but to contact them again if that doesn't happen. I've had to then contact them again a few times because the book was considered lost in the mail by PBS's system and I didn't get credit. The last time that happened, they told me they'd contact the member but to give it another 3 days (which honestly ticked me off--the book had been scanned delivered 3 days after I mailed it. I had already waited almost 3 weeks). Two days later it was marked received. I'm guessing TPTB marked it received although I can't be sure.

I'm not sure why they tell us to contact them if there's no response to the PM if they're not planning on doing anything at that point, but I do so so that there's a record of me doing everything the HC asks. I do not PM the member again if it goes lost; I can then to straight back to TPTB.

In this situation I would:

Contact the member 1 week after delivered. 

If the book is not mark received I would contact them again at least 1 week before the book goes lost.  

When it goes lost and I would contact PBS PRONTO for no response from member.  

They don't want to intervene until the book is officially lost.