Hi Btrinh,
Social engagement is about connecting with people. It’s about responding to people’s comments, asking questions, etc. The point is to create a relationship with people or create a community of people. It’s not something that has a direct connection with SEO, but rather an indirect connection. By forming these relationships/communities, it makes it so people are more likely to consume your content and then link, Like, Share, retweet, +1 etc. when you push content out (the content still needs to be good). They are also more likely to tell others about your organization. So, it provides indirect SEO benefit as well as many other non-SEO benefits.
Good on-page optimization is a must, but link building is different today. For link building, I think the focus today needs to be on doing a great job with your company/website, creating a good user experience on your website, creating great content, connecting with people personally and on social sites so you can push your content out and get people to respond by linking, Liking, retweeting, etc.
Most of the old link building strategies which focused specifically on getting a link, especially one with your target anchor text, aren’t of much benefit anymore. Some can provides some value if done while also doing natural link building. You might get some benefit from article marketing if you produce good enough content that people use your stuff. You don’t get any value from links on the article sites themselves. There is also value to contacting relevant websites and asking for a link, but that is best done by first forming a relationship with someone who works on the website and you are giving them a good reason to want to link to you (your company is good, you have good content, etc.).
The point is that Google has always wanted links to come naturally, either because someone likes your company/website or you produce great content. Up until recently, they weren’t technically able to enforce that, so people were able to come up with unnatural link building strategies that worked. Now, they’ve advanced their tech to where those unnatural strategies generally don’t work anymore.
-Kurt Steinbrueck
OurChurch.Com