What follow-up actions, if any, does the FAA take when a pilot declares an emergency in flight? This question focuses on what the FAA is required to do when pilot declares an emergency, as opposed to what a pilot is required to do. (The answer to this question is not covered in typical pilot training). 

Example: Pilot loses situational awareness and becomes lost. Pilot is low on fuel and declares emergency and ATC provides vectors; pilot turns towards nearest airport. Pilot arrives at airport and makes an uneventful landing at small airport. No other traffic was diverted, aircraft and passengers fine, pilot did not deviate from any Part 91 FARs. 

After the emergency is over, what legal, administrative, or investigative actions will the FAA or other official bodies take?

I understand that when a pilot declares an emergency the pilot gets priority from air traffic control, and may take reasonable actions for the safety of the flight even if it means deviating from part 91 regulations. 

It would appear that, so long as there was no incident or accident, no one is injured, and no regulation is broken that FAA will take a "no harm, no foul" approach. 

I'm curious if any one has source information as to what should the pilot expect: a call from ATC, an investigation, nothing, etc.? (Assume no FAA deviations, no injuries or accidents, etc.)
I don't know the FAA policy, but normally, you don't report deviations from the regs, unless asked to.  I have declared emergencies, gotten priority handing in the DCA area, and a variety of eyebrow raising events over the years.  However, the FAA has never asked me to submit a written report. Once or twice, I have gotten a call from an operations inspector, and answered their questions, and that was all that became of it.

In general, if you had a bad day, don't tell about it. Just get on the ground safely, and be polite to people, and hope that you don't have any further review.  Of course, you must comply with NTSB 830, but if you are required to do that, you have had a really bad day, and might want to get some help (aviation attorney).