While a focus on getting more Facebook friends, retweets or LinkedIn followers is not bad per se, it does not really help you early in the journey of your startup. The slogan to "Nail a Niche" is very applicable to most startups. You need to get to Product-Market-Fit, and getting close to your early Adopters and Innovators is the best way to get there. Long Tail Social media communities are where you find these audiences. This could include a Facebook group or a specific Twitter influencer. More likely, however, are you to find success in the following watering holes:
Not only do references to topics in Quora get ranked very high in Google Search results, but the Q&A format also really helps you understand the questions your prospects and customers ask during their journey. These are great pointers on how you can create content to drive them through your funnel.
A great place to get help from early adopters to get your product to "minimal viable product" state, and maybe even on a path to product-market fit. One of the SaaS Startups who I helped get "from 0 to 1", Atera, really benefited from early engagement on the Reddit platform. Gil, the CEO started engaging in the Reddit MSP community early.
This is the list of many results that you now get from a search with great "free" content on Reddit that helps Atera's prospects and customers during their buyer's journey and that drives many new organic trial sign-ups. It only needs a little bit of (genuine) engagement from the team.
3. Blogs with comments
An example is one of my favorite blogs about Software (Cloud) Platforms that have a small, but very engaged community comment/discuss a topic that could be very relevant for what your startup does. Platforms like Disqus and IntenseDebate offer great opportunities for "User Generated Content" and search rankings relevant to your solutions.
4. LinkedIn groups
Don't focus on LinkedIn followers, but engage with the specific groups of people that fit your ideal customer persona best.
5. Help the press
Sites like HARO (Help a Reporter out) are other communities to engage in with your content.
6. Technology community
Engaging with communities like GitHub are great ways to engage with early adopters and get traction with potentially your most loyal fans, who also will help you improve your product.
7. Long tail
Finally, there are sites like Tedium, focused on surfacing interesting stories that somehow never got to the mainstream.