Johnny-There is a web page on Fairfax County's site which indicates what occupancy is and what is not allowed in single family residential neighborhoods. The one rule I recall was that four unrelated individuals could share a single family home. The page has been linked in a number of earlier threads on Fairfax Underground. You will have to find that page yourself. As to airbnb operations, see the following:
http://www.fairfaxcounty.gov/code/property/property-rental.htm
If after reading the rules there are homes in your neighborhood that do not comply, you can report them to the County.