Here's a macro that provides a Twitter Follow button for a particular username:

\define twitterFollowButton(username)
<iframe allowtransparency="true" frameborder="0" scrolling="no" src="//platform.twitter.com/widgets/follow_button.html?screen_name=$username$" style="width:300px; height:20px;"></iframe>
\end

The only change from the version published at https://dev.twitter.com/docs/follow-button is that new lines have been removed.

Note that the src URL is given without a protocol (ie "http" or "https"). It's done this way so that it works on sites whether they are hosted on a HTTP or HTTPS domain. If you want the Twitter button to work while using a TiddlyWiki offline on a "file://" URL, then you'll need to manually add the protocol. For example:

<iframe allowtransparency="true" frameborder="0" scrolling="no" src="https://platform.twitter.com/widgets/follow_button.html?screen_name=jermolene" style="width:300px; height:20px;"></iframe>