Are there some minimum criteria for a field to be registered with the FAA as an airstrip? I spend an unacceptable amount of time 'flying' around the country (U.S.) via Google Earth and I'm always zooming in on airplane symbols to have a look at airfields.  In the western U.S. there are lots of rural strips that have the little aircraft symbol on them which I'm presuming come from the FAA.  Some of these fields appear marginal at best as legitimate airfields, especially the ones that are obviously part of a farm or ranch.  

My question is what level of improvement is necessary for a field to be listed by the FAA as an airstrip?  Is there some minimum length, width, slope, etc.?  Is there some inspection that is done before you can get put on the charts and are there any ongoing checks to make sure the field is still there when the charts are updated?  Does the owner of a strip assume some ongoing obligation as a result of their strip showing up on a sectional?
Good question.

14 CFR Part 139 requires FAA to issue airport operating certificates to airports that:


Serve scheduled and unscheduled air carrier aircraft with more than 30 seats;
Serve scheduled air carrier operations in aircraft with more than 9 seats but less than 31 seats.


Also:


  VFR charts depict airport's hard and soft surface runways. Enroute charts depict airports having hard surface runways of at least 3000-foot in length. If your airport meets those criteria, you may contact the local Federal Aviation Administration, Airports District Office or Aeronautical Information Management, AJV-21, at 1-(866) 295-8236 to have your airport charted.


That means for a private airstrip, it's your call as the owner to have it placed on a chart. Pretty cool.

More: https://www.faa.gov/licenses_certificates/airport_certification/