We’ve been bullish on the potential of messaging apps like WhatsApp, Line, KakaoTalk and WeChat for a while now: they’ve been not-so-quietly emerging as alternative social networking tools to Facebook and Twitter, particularly for teenagers. Now WhatsApp has announced a milestone: 250m monthly active users, which makes it bigger than Twitter based on that company’s current stat of 200m active users. What’s interesting about WhatsApp is that for many of its users, it’s a paid service: it charges $0.99 to download the iPhone app, while the free Android version switches to a $0.99 fee after one year of usage. As a reminder, some of those rivals are also growing fast: Line has 150m users, WeChat 300m and KakaoTalk 90m. There are opportunities for music in these apps too, most obviously with sales of branded stickers or emoticons (for big artists who lend themselves to the cartoon treatment), but perhaps in the future, for distribution and promotion of music as well.