None of these five men are active bloggers or columnists. However, I have been impressed enough with their theories, general erudition, or the sheer scope of their work to set up a Google Alert  on each of their names, so that I am notified if they publish a new essay or are included in a new roundtable discussion or interview.
The five thinkers whose words I value enough to follow through Alerts:
Vaclav Smil: The word polymath was invented to describe Mr. Smil. A sampling of just some (!) of the books he has written in the past four years: Made in the USA: The Rise and Retreat of U.S. Manufacturing, Should We Eat Meat? Evolution and Consequences of Modern Carnivory, Why America is Not a New Rome, Harvesting the Biosphere, and Energy Transitions:History, Requirements, and Prospects (see the full list of books currently in print here).
Every book Mr. Smil writes would be the product of a lifetime of work from almost any other scholar. Conversant in all fields and an expert in more than a few, Smil's work is dense, data driven, and absolutely necessary for understanding how the world works. His Energy in Nature and Society: General Energetics of Complex Systems changed how I think about human society and history--as can be seen in some of the most popular historical essays on the Stage.
Azar Gat: Mr. Gat began his scholarly career studying the evolution of European strategic thought and the connections between European intellectual history and the way war was actually waged. These works are considered classics in his field, but what sets Gat apart from other scholars is his magisterial War in Human Civilization. With War in Human Civilization Gat managed to synthesize the work of biologists, ecologists, psychologists, anthropologists, historians, sociologists, economists, strategic theorists, and political scientists into one cohesive whole. He does not present a general theory of war and civilization (thank heavens!), but does touch upon almost every single aspect of human society before the work is completed.
I do not agree with his take on everything, but I am more than willing to listen to his take on anything. It is difficult to take seriously the work of scholars opining on human conflict or the dynamics of human civilization if they have not read anything by Azar Gat.
Robert Putnam: Robert Putnam is best recognized for his book Bowling Alone, in which he showed how across America 'community' - or as he called it, 'social capital' - had fallen apart. The importance of social capital for the proper functioning of society is a recurring theme of this site and I have cited Bowling Alone many times here. Mr. Putnam's other work is less well known but equally illuminating. His first book, The Comparative Study of Political Elites, has been largely forgotten. This is a shame, for it offers one of the only coherent theories of elite power I've come across. His most recent book, American Grace, is the best single work on American religious practices and trends; those who write about broad social trends in American life without referencing the data inside it usually sound a little silly.
Robert Putnam has now turned his attention to the way growing economic inequality is changing American social values, practices, and capital. I can only hope he turns this work into another book; the research studies he has produced on the topic are interesting enough to deserve one.
Clayton Christensen: Mr. Christensen is the only person on this list I have had the opportunity to meet personally. He has been hailed as the most influential business theorist writing today, and his Innovator's Dilemma has been described as one of the most important business books ever written. A lot of business literature is garbage; not so with Mr Christensen! His work is both empirically valid and theoretically sound. Most importantly for this author, his models and theories about innovation and management can be easily applied to other fields (see, for example, this post I wrote last year). His How Will You Measure Your Life? is a good crash course on the various models and theories he has developed over the last fifteen years (though there explained in simpler terms than in his more academic books and articles)
I met Mr. Christensen when serving as a missionary for the LDS Church in the New England area. It was here I came across another group of his writings on how to best strengthen LDS congregations and be an effective missionary. Many of these writings are now available on the website Missionary Leaders. Mr. Christensen's approach to gospel service and his approach to life in general has impacted me deeply.
Like the other people on this list, he is a wide-ranging, first rate thinker. His op-ed, "The Capitalists Dilemma," is one of the most important articles published on American political economy in 2013. I eagerly await the book-length version to be published later this year.
Siddarth Vadarajan: I started following Mr. Vadarajan when he was the strategic affairs editor for the Hindu. This meant it was his job to write the monthly editorials on geopolitics and international affairs--he did this with an analytic clarity few Americans could equal! Mr. Vadarajan's insights were valuable on their own merits, but for Americans like myself his clear thinking and prose provides an excellent vantage point to understand how the India's policy elite perceive world affairs. For the last few years Vadarajan has been employed as the Hindu's lead editor and has thus written very sparingly (though he did manage to make it onto the list of authors for the excellent Non-Alignment 2.0). He resigned from his position at the Hindu in 2013 and it is not clear if he will be writing in the future. I can only hope that he continues to write about world affairs.
EDIT (3 April 2013): I have been informed that Siddarth Vadarajan now has a blog! It looks like he started posting new pieces during December of last year.
 I actually don't use Google Alerts, but Talkwalker Alerts for this purpose.