I had a conversation about marketing, sales, and business development earlier today with a startup founder in the Bay Area for a couple weeks for meetings and networking.
I must disagree with Omon. PR and traditional advertising is simply too expensive, too slow, and impossible to track. The research has shown that it really doesn't convert well and startups need to focus on conversions, especially to get that initial traction to attract investment dollars. We should all be so lucky to have our startups go viral, but solid marketing, business development, and sales should result in a solid customer base. Ideally, those customers will love what you've created and become advocates and evangelists who will trigger that viral effect.
Growthhacking (a term coined by Sean Ellis) is seriously labor intensive if done correctly, but definitely more than pays for itself. Otherwise, engaging a marketing consultant who has experience with online campaigns and understands the startup environment is worth the investment.
You have to identify your market. Define the personas of your customers. Or, follow Clay Christensen of Harvard Business School and use job-based marketing. Some products are better defined by the job they do than the customers they serve.
Then, figure out where your customers hang out. Some are on Pinterest while others are on LinkedIn. Choose three of the most likely social media and focus your efforts on them, along with good ol' email marketing.
Measure everything. Adjust. Test. Measure. Adjust. Repeat.
I'd suggest you do a search for Sean Ellis, GrowthHackers, Dan Martell, Peep Laja, Conversion XL, and also 500Startups Distrosnack. Subscribe to everything and read it all. You'll get a great education.
Spend some money on the right tools but don't get sucked into spending more than necessary.
Disclaimer - While I'm working to launch Personal Health Cloud, I'm still running my company, Venntive which a complete marketing-sales platform. I've been advising on online marketing and email marketing since 2001. If you'd like to take the conversation offline, just ping me.