Hi guys, please why did Twitter stop using MySQL

Last year, I tweeted "Twitter uses MySQL" and i received reponses that it's the legacy database of Twitter and that twitter had moved most of their main data from MySQL to another database, if i could remember someone mentioned Cassandra or so

And it has got me thinking why Twitter moved away from MySQL, was it that MySQL is not good enough at handling large data, or it has a bad performance when handling abyss of data like that of Twitter, or was it because it is bad at scaling when handling ultra-large (if there's a word like that :D) data

I am about to start building an app - a freelance project - that will in the nearest future start having large amount of data and user base and I am being skeptical of MySQL being the best database for the app

Please why did Twitter stopped using MySQL

Thanks