At Plurk.com we use both LightCloud and memcached. They complement each other. I.e. memcached is used for caching stuff - to reduce the load to MySQL. LightCloud is used to store persistent data - such as how many times a plurk has been read.
You could also store this in MySQL, but generally, storing key-value data is not the force of a relational database such as MySQL and LightCloud (and other key-value databases) are optimized for this kind of storage.
You could also store this in MySQL, but generally, storing key-value data is not the force of a relational database such as MySQL and LightCloud (and other key-value databases) are optimized for this kind of storage.