URL schemes are not the most comprehensive (and likely, in my opinion) way of accomplishing this tracking. Most apps do not have any kind of URL scheme.

Here's how you do it for any app on the system, with or without a URL scheme:

1. Use sysctl to get the list of active processes.

2. Filter out all system processes

3. Upload to the server for further processing.

Combine this with a silent background refresh push notification, and you can have this run ~3 times per hour without Apple throttling you!

Source: I have shipped an iOS app for a client that does exactly this, although not Twitter.