What is the key element of Airbnb marketing strategy that makes it so powerful across many markets?

I'd say the fact that people are increasingly money conscious and noone wanting to be ripped off by hotel chains or have to stay at flimsy places on a budget. Airbnb offers relatively interesting accommodation for good price. However, it is so badly made and customer unfriendly that I am expecting something easier to use to come along and take supremacy..

Airbnb has several elements that make it stand out from the crowd.  As a pioneer of the P2P sharing marketplace, the company revolutionized hospitality.  Ultimately Airbnb identified a desire for community, for the authentic experiences that are part of traveling as/with a "local".  Airbnb does not sell a product it sells and enables experiences.  As such, the company has developed some strong content tools, such as its Pineapple print magazine - meant to present a collection of the communities stories and inspirations.  

A desire for connectivity and exploration is what unifies the community of travelers, Airbnb is inspirational not aspirational - using the service oftens yields a more affordable trip than staying in traditional hotels, and virtually always leaves the customer with more vivid experiences and stories to share.  Using this material to build the experiential brand is smart and makes sense.  

The main key success factor from my point of view that Airbnb is using the strategy of user generated content that allows all citizens from all over the world to advertise for their apartments and connecting them with people who wants to rent or find apartments all over the world. 

Connected world always improves business just like if a railway / airway lines are open to a particular geography that place tends to develop over the period of time.

In the say way Air BnB have started connecting places to people which otherwise would have been a task at hand for many. Also since these are all created newly they ( AirBnb) got price advantage and huge margins to invest further in technology for higher connected places in network. 

Airbnb provides an online market place across the world. The benefits are: offering lodging, Safety mechanisms, References and reviews, Insurance, Logged private messaging system, Decline or cancellation of reservations, Secure payments, Security deposits and cleaning fees as well as Verification of identity.

Technology is developing second by seconds. Many are coming up with new online ideas. It depends upon the demand and supply of the region.

It the same as asking your friend, how much money does the King have in the bank. How do you expect your friend to know how much the king has in the bank if he/she is not the King's financial secretary.

With that said, we are able to identify the strategy Airbnb used during its implementation which was purely growth hacking because they had no money. The benefit from a flaw in Craiglist and pushed their business to the limit before it was discovered. 

Growth hacking is based on insights and data and how you use it. It also takes into consideration every activity geared at driving traffic and retaining users at any cost. With growth hacking, you never think about profits but instead satisfying the customer especially with a business like airbnb.

Marketing strategy with a higher purpose to transcend academic rules learned at business schools (demographics, psychographics, target market etc.). Airbnb experiences are about connecting people based on trust. Airbnb algorithm are powerful engines that can predict gaps in relations between "strangers" based on the brand promise and predict the outcome; constantly doing it to further the experience and deepen the competitive advantage.

Airbnb’s rise has been meteoric and much of it is down to its unconventional yet inspirationally innovative marketing strategies. It is not due to bloated marketing budgets, but a result of hard work, ingenuity and a lot of panache.