The first thing you should know is that finally Google has opened its social network site Google+ to businesses. It currently has around 40 million users, and while this is no threat in the short term to Facebook which has around 800 million active users, it's a social media platform that cannot be ignored.

Clearly the two big players in the social network world are Google+ and Facebook; not to have a presence on both is simply not good SEO sense. If you want your brand or site or company to have maximum publicity online, then you should be a part of these online social networks.
Facebook has its Like button which allows people or fans to express their opinion about certain products or sites and when they press the button, all their friends are informed that they have given something the thumbs up. This may not result in sales, but if only one sale comes from sixty-six Likes, that's better than no sale at all. Incidentally I didn't just pull that figure out of a hat; it was taken from a recent MerchantCircle survey published in August 2011 which surveyed small local businesses.
You should by now have embedded a Facebook Like button along with a Google+1 button onto the pages of your website, as the Google+1 button will, one presumes in the future be an indication to the search engine of your page's popularity and so will affect your position in the Search Engine Rankings Pages (SERPS).
Having a Twitter account which you Tweet to regularly cannot harm your position in SERPS either, in fact the more social media sites you have a presence on, the better for your business and ultimately for your rankings in the SERPS.
Some people in online marketing forget the value of Wikipedia as a social media. It ranks highly in Google's SERPS, even though it might not be the most authoritative site on any one particular topic. If you can contribute to it and get your site linked to pages within it you have got yourself a highly rated backlink.
However there is no getting away from the fact that Facebook and Google are the major league players in social networking and so in online marketing; ignore them at your peril. Google+ Circles allows businesses to categorize their fans into clusters according to age groups and gender etc., so that allows businesses to target specific groups with advertisements. There is no denying that the trend in marketing is for businesses to communicate with consumers and both Facebook and Google+ allow for this to some extent. Both sites are in the business of providing their users with so much information that they will have no need to leave the sites. This means that if your business does not have a presence in these social network sites, it will be losing out to competitors who do have a visible presence in their pages.