The site is a subsidiary of TheStreet.com, described by them as
MainStreet provides personal finance tips and advice to help consumers grow their wealth and enhance their lives. By combining lifestyle news, commentary and financial resources, MainStreet is an engaging and fun site "where life and money intersect."
Of course, TheStreet has been around since the early days of the web.
Mainstreet has a large number of contributors way more than a group blog such as Power Line or Legal Insurrection as you can readily see with a simple google. How they are compensated and whether their compensation depends on the number of hits their articles get, I do not know. I highly doubt they are anything like full-time employees. I suspect they are freelancers who get spare change in the Paypal accounts.