Should Facebook and Twitter be the only social networks to promote your brand on?

Since Twitter and Facebook are the most popular social networks, is it even worth targeting other sites in a social media marketing strategy?

Has anyone had any consistent success with other social media sites outside of FB and twitter?

Target people where your users are. Most people are not on Twitter. If you target generalist sites such as Facebook or Twitter and you are not good at targeting, you'll waste your money. It usually pays off to find smaller communities that match your product better.

It depends on your brand and with your target audience

Facebook - over 1 billion users
Tumblr - very young audience of teens and millenials
LinkedIn - works well for b-to-b marketers
Twitter - a channel for brand maintenance
Pinterest - fashion, beauty and health are so popular on the site