If I run phpinfo() it tells me that memcached is installed.
memcached support enabled
libmemcached version 0.44
Session support yes
igbinary support no
sudo pecl install memcached
It's possible that it didn't install correctly or, as you say, you might need to add the following to php.ini:
extension = memcached.so
But since you say it shows up in phpinfo() it seems more likely that it just didn't install correctly. Were there any errors in the output during installation?
Also, to state the obvious, make sure you've restarted your server.
I wrote up a tutorial on getting memcached running on Ubuntu, which I mention not to self-promote, but just in case it's helpful.