What is social capital?
It is the respect and trust you build with individuals as a result of working with them over time. Like financial capital, you can ‘invest’ social capital towards specific objectives. It results from implicit, ongoing networking (interactions over time with colleagues.) See also: Networking vs Social Networking vs Social Capital where I try to tease out the distinctions, and Networking to develop social capital where I look at how to develop your 'net worth' in this critical resource.
I have examples in the career development section of this site; but there are many more. Robert Putnam's book "Bowling Alone" documents the elements involved in social capital, and the results. These may range from personal health advantages, to lower crime rates, or more kids going into science, technology, engineering or math (STEM).
You may invest social capital to start a standards committee, publication, conference, game club, users group -- or find a team to engage with "Engineers Without Borders" to bring computing efficiencies to help a village in Africa, or to help the elderly in your community develop their World Wide Web skills, or coach a FIRST Robotics team. The big difference: most expenditure of social capital actually gets you more trust and respect -- you acquire more social capital by doing these things, it is not a zero sum game.
I have colleagues encountered in standards meetings who have started their own companies, others who have contributed to combat leukemia marathons, others who have gotten involved in local schools to support STEM education, and others who have headed off to Africa to help folks in a totally different culture.
If you consider how you 'link' to other persons, the more links you have and the more diverse they are, the easier it is to draw on them (and for them to draw on you) to address something that might be of common interest. There are reasons why "executives play golf" and local merchants join the Lions, Chamber of Commerce, etc.
This is not new. Research shows that 19th century accounting firms in Amsterdam with greater social capital connections stayed in business longer than those without these connections.
Is it just IEEE Computer Society that can help you build these? No, participation in your religious community, your kids school community, your town council, clubs, and a variety of non-governmental organizations can build similar connections. But, from a stand point of career and innovation (which are all part of the mix), your professional societies provide unique advantages.