If you'd like to see your Google Analytics sessions broken down by paid versus organic sessions, you can utilize filters within the Custom Metrics in combination with Custom Reports to see this data.
Step 1) Create a Custom Metric for your Organic Sessions
In this example, we're creating a metric that will yield all organic sessions from Facebook specifically. To create the metric, launch the Custom Metric builder either via Custom Reports or the Key Indicators tab, and add in Sessions from Google Analytics as your starting metric.
From there, on the "Filters" tab, you'll need to add in two filters:
Source CONTAINS Facebook (or whatever source you'd like to see sessions for)
Medium IS not set, referral, or organic
These three mediums should each be selected when you're creating a metric for organic sessions.
Don't forget to save your metric at the bottom of the Custom Metric builder once you're finished setting your filters.
Step 2) Create a Custom Metric for your Paid Sessions
Repeat the above process, but adjust your filters accordingly to account for paid sessions:
Source CONTAINS Facebook (or whatever source you'd like to see sessions for)
Medium IS NOT not set, referral, or organic
Step 3) Pull both Custom Metrics into a report
Once you've created and saved both of your Custom Metrics using the steps above, pull both of those metrics into a Custom Report to see the breakdown. You can also add Sessions from Google Analytics to this report to see the total sessions.
Keep in mind that you can adjust the filters within the Custom Metrics (or the report itself) to further segment this data.