The amount of websites using these applications has been growing exponentially since their conception. CoinHive even released this statement on their blog after their initial release:
"See that little hill on Sep. 14? We had an Oh shit! moment right there. A few sites were actually using our service and collectively mined at 100K hashes/s. We have since peaked at 13.5M hashes/s – a quite respectable 5% of the global hash rate of the Monero blockchain.
In just one week we scaled from one lonely server to 28 WebSocket proxies, 6 web servers, two database servers and two VPS doing maintenance work. We had countless performance issues to fix and a few sleepless nights, but we are now handling 2.2 million concurrent WebSocket connections quite comfortably."
What you may not be noticing is the amount of applications that are spawning up around these new available streams of revenue. If you are familiar with Cryptocurrency Faucets (websites that give you small amounts of cryptocurrency in a certain time frame) you will notice many of them have been adding optional "Miners" that will increase a user's payout if they stay and mine for the Website Owner. FaucetHUB, one of the leading micropayment systems for Cryptocurrency Faucets, enables every user to mine if they choose, paying them minuscule amounts of Satoshi per so many hashes, but they also lure the users with a level system, making them feel as if they are being productive while also earning income. Mobile Games are even being released where users can opt in to use their phones processing power for in-game rewards, doing away with the old micro-payment system used in so many games.
Why are these people choosing to use this way to mine? There is no doubt at all you will profit more from just using XMR-Miner since you are paying the application creator (i.e. coinhive) on top of whatever the website owner is also taxing, but many people just like the simplicity that these websites offer. The gamification of the system is alluring to users, and one can just look to other models such as xbox live gamerscore, steam levels, etc, etc for proof of concept; People just like to feel like they are doing something productive.
Not all people are over-joyed with the rise of mining application. Google Play App developer Alexey Khripkov recently removed all crypto programs from his applications when users reported them as malware. He has since released "Reward Digger", which gives in-game bonuses for Mining.
There are mixed feelings about the use of mining applications in the web. Use it without notice and you are going to make people angry, but use it correctly and it can be an amazing income stream. I have a feeling we will start to see more of this in the future, with sites opting to allow users to choose between an ad-free experience in exchange for their processing power.