What are the best strategic options for building critical mass of a new social media site?

Focus initially on 1-2 user groups that have the greatest need for your solution, and a sense of urgency & motivation to change.  These users must become aware of your solution, like what they see, be willing to talk about it and persuade other buyers / influencers to get on board. 

Events that bring the entrepreneurial tech community together, like the recent TechWeek Chicago and SXSW, are great opportunities to showcase new ideas & get people talking about them.  The tech press, local tech groups / association newsletters, VC & subject matter blogs are all important channels to communicate with individuals who want to see & understand new, innovative business models and to reach the entrepreneurial community at large. 

The goal in raising your startup's visibility in the entrepreneurial space is to get lots of people telling your target users about your solution and your brand.  If you haven't been specific enough about who your target user is, they won't know who to tell.   

In my experience, focus creates momentum and shortens the distance to sales.  Even for startups that offer technology solutions that can go anywhere--have broad market appeal--my recommendation is to build critical mass in 1-2 key market segments initially and expand outward from there.  While your  business is deeply penetrating these segments, you can be learning more about the buying dynamics of other potential buyer groups and prioritize accordingly.