Long ago the IRS requested to Coinbase to send available information about US citizens, for tax purposes and other stuff. Coinbase was able to refuse but seeing how big it becomes with the time passing, perhaps in the near future, it won't be able to refuse and will have to obey.
They weren't able to refuse completely. They fought the IRS in a year long court battle. In the end,
they handed the IRS data on over 14,000 traders:
A California federal court has ordered Coinbase to turn over identifying records for all users who have bought, sold, sent, or received more than $20,000 through their accounts in a single year between 2013 and 2015. Coinbase estimates that 14,355 users meet the governments requirements. The full order is embedded below.
Now, if you surpass $20,000 and 200 transactions in annual volume, they send the IRS
a 1099-K about you. Kraken and Gemini do the same, and others will likely follow. I interpret this almost as a direct response to the Coinbase case.