By DAVID KOENIG, AP Business Writer Home-sharing site Airbnb posted a $3.9 billion loss in the fourth quarter of 2020 as it suffered from the pandemic downturn in travel and recorded one-time costs for becoming a public company. In results released Thursday — Airbnb's first as a publicly traded entity — the company took a charge of $2.8 billion for stock compensation related to the IPO. A year earlier,