A good marketing business plan means putting your message out there in as many places as possible and not get stuck in the same pitfalls that you see happen to people that bank their livelihoods on a position in Google.
To have a good thriving online business you need a balanced mix of SEO, PPC, directories, social media, etc.
Anything less is either narrow minded, lazy, or both and a recipe for massively critical failure in the event something happens to your primary traffic source.
More importantly, you might need to custom tailor your message for each platform because what works on Facebook may not work on Pinterest or Twitter, etc. Always try A/B testing to find the sweet spot for each platform.