When you start writing filters you may only have one Data Source enabled. However, as you start working more with the platform you're bound to enable many sources and you might also work with many Managed Sources.
In fact you might well be surprised by the amount of data returned from a filter because you forget that filters by default operate across all of your enabled sources!
You can easily control which sources to filter against using CSDL.
Filtering by feed(s)
For standard data sources / feeds, you can make use of the interaction.type target:
// Just Facebook source interaction.type == "facebook"
// Data from any of these sources interaction.type IN "facebook,twitter,tumblr"
// Exclude Facebook data interaction.type != "facebook"
Filtering by Managed Source(s)
For Managed Sources, make use of the source.id target:
// Just data from source with ID "1234567890abcdef" source.id == "1234567890abcdef"
// Exclude data from source with ID "1234567890abcdef" source.id != "1234567890abcdef"