There is no measure of the number of full nodes. The number published by many websites is the number of listening nodes, instances of software which is advertising itself as available for connections on the Bitcoin network. The term "full" refers to "fully validating", as distinct from non-validating or BIP37 SPV. To be listening you do not need to be validating, or even a Bitcoin node at all (a proxy connection is indistinguishable from a real node).
If you want to know about the number of listening nodes, bitnodes.io does track numbers for these though they appear to be easily manipulated at times (see the spike around September 2015).