In Comments, Ravi mentions an aspect of this I hadn’t considered:
It is even worse than it appears for Twitter. The Google+ APIs are coming. When that happens, third-party Twitter clients will start baking in Google+ integration, especially because Twitter has been kicking their developers all year.
What does Twitter do in response?
Do nothing? The bleeding increases. Backbone users, after all, are big fans of third-party clients.
Kick developers who dare to integrate with Google+? That sounds like it would generate a devastating backlash.
Buy more clients / pay developers to favor Twitter? That doesn’t sound sustainable.
We’ve seen how popular clients made Twitter. Many people used a client instead of the Twitter site.
The same thing could happen with Google+. An app could come along that would bring sense to its UI/UX and that could be a tipping point.
This Is What Twitter Does Not Want To Happen
This Is What Twitter Does Not Want To Happen, Part Two
This Is What Twitter Does Not Want To Happen, Part Three
What A Twitter For Grown-Ups Could Look Like
Why Twitter Should Buy webdoc If Apple Won’t