Drupal - drupal.org: Free open source content management system with social media capabilities and a vibrant support community. It can have a steeper learning curve than other systems but it is the superset of joomla and wordpress: it does all what they do and much much more. Since you are looking for developers then the learning curve may not be an issue as you are not imposing it on yourself.
Your requirements:
- users can register and provide info about what types of yoga there are interested in. Drupal: users can put this on their profile page - there are several plug-in modules in Drupal that provide profiling.
- users can interact with other geo local users. Drupal: forums, blog comments, posting content, "stories" etc.
- if a user is a yoga instructor, then they can advertise to other local yoga users. Drupal provides event publishing facilities, for example
- show local yoga places to users: via story content type, use of tags to identify and describe the place e.g. what facilities it has, location etc.
Comparison of Drupal, Wordpress and Joomla: http://www.compassdesigns.net/joomla-blog/How-to-Choose-Between-Joomla-Drupal-and-Wordpress.
See this book: Drupal Social Networking: http://www.packtpub.com/build-social-networking-website-with-drupal-6/book
Download the drupal package as instructed onto your hosted webspace. Drupal works very well even with the cheapest shared hosting.