That's certainly something to remember. But there's another way in which the two online realms are closely related. And that is that people use individual social networking sites as "little Googles" in themselves. Just as you should apply SEO principles to your site to be found by people typing keywords into a search engine, you should also think closely about how to be more findable by those looking for others to circle, friend, follow and send connection requests to.
To this end, you should put your chosen keywords into your social profiles. Of course you can only choose a couple of them on Twitter because you have so little space. But you can implement this more fully when creating your Linkedin profile. (That said, you should go overboard either. Don't endlessly repeat the same keywords over and over again. Just as Google will punish you for this, so will Linkedin.) Basically make sure that you choose several good, relevant keywords -- particularly ones related to your skills, qualifications and location -- and sprinkle them throughout your profile text. And the longer and more comprehensive that profile is, the better.
Search the professional network for a popular keyword and you'll see this phenomenon in action. The profiles that come up first inevitably have comprehensive profiles comprised of thousands of words.
They also have a lot of connections, which is of course another powerful factor. Just as you want your website to have numerous backlinks from related, well regarded sites and blogs to help lift you in Google's rankings, you should aim to build many quality connections with other users within the "walled gardens" of social networks.
You should also do lots of liking and commenting on other people's updates. And participating in group and community discussions is a must. All these things create a matrix of references between you and others, and between sections of your own content. They all tell the in-house search algorithm about your quality and relevance as a user. The more you do this, the higher you go in search results.
Here's a recent example: I hadn't logged in to Linkedin for months. My profile was threadbare. Not surprisingly my rankings for keywords I'd chosen were very low. I was getting a profile view every couple of weeks.
So I added some content to my profile, including details about this site. I got active on several of the groups I'd joined, as well as writing some comments on articles published on the site. I interacted with some of the people in my network. Not only did I get some new connection requests, but under "Who's viewed your profile" it said that my rank for profile views had improved by 40% in the last month.
While Linkedin will give you an actual number to quantify the effect of this activity, other sites don't. However you can still be sure that such activity will be used as a factor in the site's search algorithm, and will help you substantially.