This is a topic that is important to us at Cisco. We believe the number of internet connected devices reached 8.7 billion in 2012. There are a number of estimates out there by others, but they are generally in the eight to ten billion range. This number would include traditional computer devices, mobile devices, as well as the new industrial and consumer devices that we think of as things.
You can get background and references on that number in a white paper by Cisco's IBSG group: http://www.cisco.com/web/about/a... Dave Evans on our team works with market researchers to sort these numbers out. IMS Research is traditionally a good source for this sort of data.
I presume your question is specifically about the quantity of 'things.' To figure that one out, you need to define things. The definition is open to some debate as there is a fuzzy line between the computer / non-computer classification, and some of the things out there look a lot like computers - is an industrial computer in a factory a thing (maybe), how about a micro-controller in a sensor (definitely)?
Another delineation is how the things are connected to the network. Many people are excited about M2M, or Machine to Machine communications. But when the Service Providers use this term, they are typically referring to only the mobile use cases for connecting to things. While mobile is the future, lots of things are connected via fixed networks. AT&T for example has a long standing business connecting railroad crossing sensors with legacy POTS connections.
The final complexity is how do you count aggregation layers - for example my home security system has twenty sensors, which talk to a controller, which connects to a router dedicated to the system. Is that one, two, or twenty things connected?
In any case, if you use the broad definition of things - just excluding the obvious computer, mobile devices, and so on - then less than a third of the total number of devices are things, with most of those being consumer, industrial, medical, or military. So, the short answer is two to three billion things as of today are connected to the internet.
Despite all these things already on the network, we estimate that more than 99% of physical objects that may one day join the network are still unconnected. There will be about fifteen billion devices connected by 2015, and around forty billion devices by 2020. Think about what happens when we connect all of those unconnected devices. At Cisco, we call this the Internet of Everything - the connection of people, process, data, and things - enabling the applications of tomorrow.
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