You are asking a question that is still evolving and for many people is how they gain a competitive advantage over others.

Three main opinions exist.

Social Media Helps Gain Business

This camp is all in, throwing feeds out to multiple services like tumblr, Twitter, Google+, Facebook, etc. Many times they post to one service and have it trickle over to the others. They also can use software that essentially handles the multiple posts for them and also helps to fix formatting or style issues (ie. Twitter length limits).

Facebook has the most traffic in this market. You will see "Fan" type pages, business pages, "Likes", contests (although this is somewhat limited by the terms now), photo albums, newsfeed items, and so on. I know many photographers that praise the time they spend on Facebook as essential to the continuation of their business. If it wasn't for the time they put in early on in the business years on Facebook - they wouldn't be successful.

Social Media Wastes Time

This group has either tried out social media or never even stepped foot in the realm. They may have a Facebook page because someone told them they should, but they rarely update on a schedule, or update at all. They don't worry about "marketing" using these mediums because they don't think that any business will come from it. It may not, that may not fit your demographic.

Twitter is one place that I have seen many photographers spend countless hours "following", "direct messaging", etc to gain business - only to later decide that it is detracting from taking photos/editing photos/ and gaining clients. It is easy to spend hours a day on social media, and people in the "wastes time" group have decided it is not worth it.

Social Media is used by my business...

...but we aren't sure of the impact

Many photographers in this group are not experts per se on social media. They have a Facebook page for the business, and check up on it regularly. But they might not consider it a great place to gain business or add new customers.

How does one monetize what time they spend on Twitter? That is a very hard question for most people to answer. So many people who use these services fit into this category. They use social media because everyone else does, and it doesn't seem to hurt anything at least.

Full Circle

So the main question you are asking is, "how does social media change how photographers present portfolios and gain business, as opposed to having a stand alone website?"

Most photographers that are active today take a multi-step approach, or a don't put your eggs in one basket style. Not only do most professional photographers have a well established professional portfolio website with an associated domain name - but they also have a presence that is connected on the social media websites such as Google+, Facebook, and Twitter. You can do this by adding a "Like" or "+1" button on your portfolio website. The same goes for the other way around. Make sure your Facebook business page has a link to your portfolio site, and add a comment below images with a link back to the full size images or the main site.

Google+ is a very new service, and I have read figures that show how segregated the user base really is currently. The activity is mostly taking place by a few tech related "celebrities", and the users who read that are mostly males. Facebook has a much, much wider audience, and already has a much larger impact on the photography industry. Some photographers do choose Facebook as the only medium to present their work, but I think that is the minority, and will be for some time. Having a stand alone website really gives you the opportunity to show off your work, artistically how you want, and have full control. Also, it shows potential clients that you are serious, and didn't just setup a Facebook page after you got your first DSLR.

Overall, social media is another tool. Currently it is not at odds with professional portfolio sites - but this is gradually shifting, and in some time we could see professionals abandon the current model.