You are going to need the Safari URL schemes and add them into your contact. Create a new contact and place the URL scheme for apps you would like to be opened using Siri into your new contact.
Let's say you want to command Siri to open Facebook, Facebook profile and Facebook friends, the URL Schemes you need to add into contact are:
- fb://profile – Open Facebook app
- fb://profile – Open Facebook profile
- fb://friends – Open Facebook to view the friends list
And here is a video demo showing Siri is used to quickly open apps.
You can visit this link to get more references of URL Schemes.