To be honest I thought everyone was using it... It surprises me that its not that big in the US. I would go as far as to say that its the 1st app most people I know would download (Europe, Australia, South America)
Some of this has been said already & some not so Ill just go ahead anyway
The reasons its so popular is:
- Its much faster & easier to use than skype & viber
- You can send photos & videos
- You can record & send voice messages similar to a 2-way radio/walkie talkie
- You can create groups for family, friends, work-mates where everyone within the group can rec. & share pics, vids, voice msgs
- No ads (yet)
- Available across all platforms