user10238: The bitcoin UTXO set appears to have stopped growing on 2017-06-3 after 5 or more years of nearly constant increase. What are the reasons (or hypotheses) why?
Charts below:

2012 to 2017-07-11 https://blockchain.info/charts/utxo-count?timespan=all
60 days to 2017-07-11 https://blockchain.info/charts/utxo-count?timespan=all



user5406: For the largest part of 2016 we saw rising fee levels. This seemed to bode that a large number of low-value UTXOs would soon become economically unspendable due to their value being outpaced by the cost of creating a transaction input to spend them.

This has strongly discouraged people from creating small outputs.
Use cases entailing microtransactions had reduced volume.
Some large companies have taken notice and started consolidating funds.
Fees hurt enough that wallet developers and companies invested in some coin selection improvements. 


After writing this I notice that my answer is pretty much echoing alcio's. Kudos to him, his answer came first.

user15715: It's hard to know for sure what's been going on. My guess would be it's a mix of :

Better coin selection algorithms used by large wallets (exchanges)
Increase in fees pushed some inefficient services to stop creating uneconomical outputs (low values). Those services would be gambling/faucets/etc.. They might now be forcing withdrawals to be above a given amount
Some services consolidating their wallets since fees have lowered a bit from the last few weeks.


10238: <p>The bitcoin UTXO set appears to have stopped growing on 2017-06-3 after 5 or more years of nearly constant increase. What are the reasons (or hypotheses) why?</p>

<p>Charts below:</p>

<ul>
<li>2012 to 2017-07-11 <a href="https://blockchain.info/charts/utxo-count?timespan=all" rel="noreferrer">https://blockchain.info/charts/utxo-count?timespan=all</a></li>
<li>60 days to 2017-07-11 <a href="https://blockchain.info/charts/utxo-count?timespan=all" rel="noreferrer">https://blockchain.info/charts/utxo-count?timespan=all</a></li>
</ul>

<p><a href="https://i.stack.imgur.com/zQmzq.png" rel="noreferrer"><img src="https://i.stack.imgur.com/zQmzq.png" alt="screenshots of charts"></a></p>
 5406: <p>For the largest part of 2016 we saw rising fee levels. This seemed to bode that a large number of low-value UTXOs would soon become economically unspendable due to their value being outpaced by the cost of creating a transaction input to spend them.</p>

<ol>
<li>This has strongly discouraged people from creating small outputs.</li>
<li>Use cases entailing microtransactions had reduced volume.</li>
<li>Some large companies have taken notice and started consolidating funds.</li>
<li>Fees hurt enough that wallet developers and companies invested in some coin selection improvements. </li>
</ol>

<hr>

<p><sup>After writing this I notice that my answer is pretty much echoing alcio's. Kudos to him, his answer came first.</sup></p>
 15715: <p>It's hard to know for sure what's been going on. My guess would be it's a mix of :</p>

<ul>
<li>Better coin selection algorithms used by large wallets (exchanges)</li>
<li>Increase in fees pushed some inefficient services to stop creating uneconomical outputs (low values). Those services would be gambling/faucets/etc.. They might now be forcing withdrawals to be above a given amount</li>
<li>Some services consolidating their wallets since fees have lowered a bit from the last few weeks.</li>
</ul>
