I've just updated and those buttons are gone. Any ideas how I can have these buttons back?
There is a duplicated question over here but it was closed as off topic.
lastfm like buttons
The version in 12.04 has a plugin to LastFM built in - however as you have found, no longer has the like buttons.
Fortunately, a third-party developer has stepped in to provide this;
called lastfm-extension the plugin:
The homepage has the installation instructions.
As per my answer:
I've packaged this together with other optional plugins in my PPA.