Update for Swift 4 and iOS 10+
OK, there are two easy steps to achieve this in Swift 3:
First, you have to modify Info.plist to list instagram and facebook with LSApplicationQueriesSchemes. Simply open Info.plist as a Source Code, and paste this:
<key>LSApplicationQueriesSchemes</key>
<array>
    <string>instagram</string>
    <string>fb</string>
</array>
After that, you can open instagram and facebook apps by using instagram:// and fb://. Here is a complete code for instagram and you can do the same for facebook, you can link this code to any button you have as an Action:
@IBAction func InstagramAction() {
    let Username =  "instagram" // Your Instagram Username here
    let appURL = URL(string: "instagram://user?username=\(Username)")!
    let application = UIApplication.shared
    if application.canOpenURL(appURL) {
        application.open(appURL)
    } else {
        // if Instagram app is not installed, open URL inside Safari
        let webURL = URL(string: "https://instagram.com/\(Username)")!
        application.open(webURL)
    }
}
For facebook, you can use this code:
let appURL = URL(string: "fb://profile/\(Username)")!