Hi, I'm a two-time best selling author and have written a book on Social Media called "Social Media Isn't Social If You're Shouting!" I've also appeared on TV, local radio and in various publications discussing social media and local small business marketing strategies. Using these strategies I built a local b2b networking group from 0 members to over 3000 in just 18 months. Today (using my strategies) the network has more than 5000 members and for the most part it was built using free marketing and social media messaging.
To answer your question: Yes ... of course it depends on what you imagine your social media agency might involve. But if you start small and offer a single simple service (like writing and posting social media updates for Twitter, Facebook, Linkedin and Google+) you can create a sustainable monthly revenue. You can also outsource the actual writing very inexpensively and managing a client with this level of service requires very little time - about 2 hours/mo.
I know because that's how I started mine. Our clients pay us a small flat fee of $499/mo (which we charge to their credit cards on the 1st of the month -- unless they cancel by the 15th of the previous month) and then we write and publish their social media updates.
If you'd like to see how we offer that simple service review my sales page at https://jamesburchill.leadpages.net/cme-sp/ and for the bigger picture, visit my main company site http://bfmx.ca -- they both explain our offer and what we do.
If you still have questions about how our program works and can easily be launched feel free to setup a call with me. I'll explain how you can setup a similar service and offer it to your local business community and how it's not too hard to get 4 to 6 clients all paying you about $500/mo.
For some people that's life altering, for others it's a start ;-)
Whatever you do, congratulations on choosing the e-shift from employee to entrepreneur, we need more people like that.