When used properly, social media can be an excellent tool to help you build sustainable, high-quality backlinks to your content.
When using social media, it's important to actually engage your audience, not just spam them. These networks are all about interaction and sharing, so you need to actually interact and share valuable material and content. The first step to doing this is to identify your target audiences. Your audience may not be one specific group, but may also encompass related demographics.
For example, say you are running a website all about wedding planning, vendors, venues and products. You're primary audience will be newly-engaged women. You also want to target their friends who may be bridesmaids, their parents, professional wedding planners and parents of the groom. Don't try to target too many sub-groups, however. Identify one or two of the largest and focus on targeting those groups alongside your primary group. Finding your target audience and its sub-groups may take some digging, but there are a range of SEO tools available to help you.
After you've identified your audience, you need to focus on building a sustainable, engaging presence on social media networks. Don't spread yourself too thin, however. Rather than dropping content on LinkedIn, Facebook, Twitter, Google+, Reddit, Squidoo, etc. etc, pick two or three sites and really focus on building your presence there. The larger networks are recommended, particularly LinkedIn because it tends to place high in the SERPs.
Once you've chosen your networks, make a human face for yourself and engage your followers and fans. This means sharing not just your links, but also asking questions, responding to others' comments and posts and re-posting others' links in your network. Active engagement is vital to gaining back links with social networks. When you engage, you give people a real reason to be involved with your business and to share it with others. This active engagement is called link bait and will help you build a network of links to your content over time.
Besides gaining followers and creating link bait, social networks are a new way to contact link prospects directly. Email is still viable and a great way to contact colleagues, but using social networking can help you look more casual and friendly. It's more memorable, and if link prospects are already in your social networks, it's more than likely they trust you and your content. When you've established a relationship, freely shared some of their content and links, then shoot them a casual message asking if they'd like to share yours on their websites, blogs etc.
Social media is a powerful tool and it can be an immense help when you're running a link building campaign. Just remember to use it in the way it was intended, as a way to build relationships, and over time you'll start to see results. Although the links aren't direct, social media networks are the best way to connect online today.