The amount listed on your 1099K are your "gross earnings". However, for tax purposes, you should only be taxed on what you are "paid out", or the amount you see in your transaction history.
You'll need to subtract (get a deduction for) both host fees and adjustments/refunds on your tax return. You can find these totals by going into Transaction History > Gross Earnings > Download CSV. I had to convert mine to a Google Doc before gaining full functionality.
Once you've downloaded the spreadsheet you can easily total your host fees for the year by highlighting that column. Adjustments are a little harder, you'll have to search for them individually as they are not grouped together.