Yoree Koh reporting at The Wall Street Journal:

Twitter said it has 241 million monthly active users the last three months of 2013. Twitter defines a monthly active user as an account that logs in at least once a month. By Twitter’s standards, a person does not have to tweet to be considered a monthly active user.

I would think active users tweet and passive users only read. If I was considered an active user of every platform I sign into in a month, I would be considered far more active than I really am.