With the introduction of mobiles in the market the usage of wifi increases very much. As you can see in the market ipad has also version with only wifi connectivity and other versions with wifi support also. Now isp also offering wifi support for mobile users which increase in wifi connection.
As you have seen in the above image south korea has highest number of users who uses wifi. United states has also good ratio of wifi users. Wifi is also cheap as compared to 3G and 4G connections. Wifi performs the same task as to see websites but has less speed in connecting. So for a middle family wifi connection is better than any other connection for his ipad and iphone and other mobile sets in the market.
The download speed of wifi is also not as good as other connection. And you cannot see high quality videos in wifi. The speed of wifi depends but I think that wifi has very good speed available still now. In underdeveloped countries 8 out of 10 people use wifi for mobile and ipads. China has still very less users using wifi but there is looking great increase of wifi users in china.