Scientists have never been quite "social". Actually, the stereotype of scientists in their labs, enclosed, thinking and finding stuff with almost no social contact is not far from reality.
So why social networks for scientists? and I mean any scientist in any branch of science (but for our interest I will be talking about basic or clinical researchers.
Why would they want to stay connected to Facebook or Twitter to update their status every hour?
Reasons are plenty but the key word here is: scientific communication.
Not that all scientists as individual should be “social” (although it might be desirable) but I think their knowledge can both be shared and increased by receiving news and ideas from other scientists. That's the part that should be "social. " This junction and exchange could bring benefits not only for themselves but for society.
 
A few features of social media for science may be:

• Information exchange. I´m not talking about ideas being stolen or that they should live with the fear of being plagiarized. I am talking about a network to increase the exchange of questions about methodological procedures, statistical and laboratory counseling as well as exchange of references and cooperation on a project online if they decide to team up. This is happening as we speak but it needs a little more diffusion.
• In the healthcare profession is known that the clinical investigator sometimes interacts with different patients with devastating chronic illness such as amyotrophic lateral sclerosis, among many others. Many of these patients already interact with each other on popular social networks such as PatientsLike Me. The researcher can create and manage these areas and even interact with their patients.
• As a technology platform. Social networks are already part of information and communication technologies (ICT) used to coordinate multinational clinical trials. Epernicus network is an example.
• As information filters. Many social media gurus anticipated that lists of selected topics in Twitter will replace and virtually "eat" the RSS readers (atoms) and as for now they work as essential filters for the creation of highly specific lists. As an example, I can, as a scientist in the stem cells arena, create a list with the hashtag (symbol #) #stemcells and/or add certain scientist (science tweeple) and a respected journal like @naturematerials and they will daily “feed me” on the subject of interest; in this instace I could reply, meet people with the same interest, exchange ideas, information, projects and, why not? make some friends.
• Exchange and management of bibliographic citations. Some great reference management applications (and already in the clouds) exist such as Mendeley, Connotea, or Citulike and act as social networks for sharing our personal bibliographic database.

Some social networks for scientists:

LINKEDIN
Of the most used in Mexico and Latin America by professionals, not just health, but all branches of education and professions. You can share status updates, uploading your curriculum, work experience, education, etc. You can add a portfolio, slide shows from slideshare, among other neat functions.

SlideShare
To share presentations we use in our lectures or if we would like to create blog posts from slides, SlideShare is the best option, as it has many ways to share and receive information.

FACEBOOK
This network, the most famous of all (if it was a country, it would be the third most populous on earth) can also function as network between scientists and all kinds of professionals. You can share videos, documents, links, create groups, including all functions that are already known.

TWITTER
In 140 characters you can say a lot. A simple tool, ideal for complicated people like us. It can be adapted as an RSS reader if we create lists of people or institutions who tweet their science news. Just go to the Twitter website on the section "who to follow" then go to the science suggestions and you will see a number of interesting people in science and technology. Or we can be more specific, and use the site search engine; for example, just type "stem cells", and a list of interesting suggestions will appear.

RESEARCH GATE
One of my favorites, it is dedicated to save and share your published work; you can create or join groups of common interest and interact with experts in your area. You could say that is the Facebook of scientists as you can write your status update, meet friends and create groups.

SciVee & SCIENCESTAGE
Want to share your research work through a video that explains what you did and how you did it? Then this network is for you. Even if you also want to share a conference or a scientific post, or browse for information, SciVee is good.

ACADEMIA.EDU
Similar to ResearchGATE and others where you share your literature and publications making friends and groups.


BIOMEDEXPERTS
A network that has over 1.8 million researchers who have a minimum of 3 publications in pubmed. Sorry to say the customer service is rather bad (in my experience).

TILT
From the creator of Tripdatabase comes a social network for health professionals where they can share interesting links and a quote in a few characters that says: “Today I learnt that...”

Doc2Doc
After declaring the obvious that I blog here I can only say that this professional networking community for healthcare professionals worldwide from the BMJ group is an excellent way for doctors to communicate in forums and blogs, meet some friends and communicate with them in private or public messages.

(... some reference managers that also work as social networks and or bookmarking managers ...)

Mendeley
In my case it is the one I use because it is compatible with chrome and firefox browsers (as opposed to Zotero who only works on Firefox) and a desktop version for downloading to your computer and synchronize with "the clouds."

CiteULike
Wonderful service that lets you store, share and manage references that you bookmarked as you navigate.

Connotea
Difficult to choose between this and CiteULike. They are both on most high-impact journals. When surfing a journal just look for the button to add to Connotea or CiteULike.

If you think other social network should be on this list. Please let me know.