Here is the probable and rational reasoning of node drop...
The drop in nodes is explainable by precisely ONE thing, which some tech deep developers appear to have little knowledge of:
Ease of use.
Users are running less full nodes simply because they don't have to. Wallets like Electrum and Mycelium are far lighter on system resources, and quicker to get started with. The same would be true if the block size was cut to 100KB. I could run a full node on my current system, but screw that. Even 5% of system CPU going to some background process is unwanted on my day-to-day machine. The same for even a paltry 5 GB of blockchain data which I could be using for cat pictures. Running the full client of a brand new copypaste altcoin is still irritating compared to using Electrum.
Businesses are also running less full nodes because THEY. DON'T. HAVE. TO. Which is quicker: figuring out how to install and configure a bitcoind, or just calling some blockchain.info javascript API or whatever? The latter, by a mile, for the majority of developers. Perhaps not for a Bitcoin Core dev, but most just want to make some cool app and aren't viewing everything through some ultra-decentralisation ideology.
The drop in nodes has nothing to do with block size: The full client has never been a lightweight piece of software, and the majority of people only used it because there was simply no alternative.
If anyone wants to actually understand Bitcoin's decentralisation, they might try making some kind of comparisons with the Tor network and number of relays or exit nodes and its usage. (Twice as old as Bitcoin, widely used, ~6000 relays which can be run by pretty much anyone, ~1000 exit nodes running one of which actually paints you as a target for law enforcement bother.)
Source:
http://www.reddit.com/r/Bitcoin/comments/383sbw/the_drop_in_full_nodes_is_explainable_by/