are you kidding ? users will never pay for twitter or facebook. As soon as facebook/twitter is a paid service people will leave it and find for alternatives.
For paid APIs that's a different story but nothing is eternal on the internet.
If payment is made mandatory for those apps, it'll kill them. But making payment an option that enables more features or services would be a great business model for them (and less prone to backlash than selling users' privacy or plastering your site with intrusive ads).