I’d say it really depends on what your specific goals are for using any one of the the growing number of social media outlets.
Facebook provides fantastic insight regarding the clientele you’re actually reaching (and you potentially have access to over a billion users). Tumblr is great for SEO but is used primarily by younger demographics, so if that’s not your target audience it’s not as useful. IDEAFit seems to be great for connecting with other health and wellness professionals as well as helping you boost your credibility if you manage your presence correctly.
In sum, I feel there are reasons to engage in certain platforms for different reasons so I don’t know that I have a favorite per se.
Ron, do you have a particular target audience in mind? If so, what channels are you currently taking advantage of to reach them?