One important thing that the Asian messengers have executed better than the others is monetization of IM, something which has eluded Western messengers for a long time. To this day, FB still doesn't charge $$ for its stickers. Meanwhile, Line reported earnings of $600M+ and WeChat $900M+ from stickers/games/ads last year [1]. I think that's what has FB's attention.
[1] http://www.nytimes.com/2015/03/26/technology/personaltech/me...
Disclaimer: Former Meebo engineer