What do you think of a good database for a microblogging web application?

What is the good database for a microblogging platform? First relational (SQL) or not (NoSQL)?

• MySQL
• PostgreSQL
• MongoDB
• Apache Cassandra

I know there's nothing called "a good database", but hey, I'm just asking for your opinions! So, what do you think? And if you have another suggestion, something like graph databases, no problem, make yourself home!