Apple’s iMessage is both one of the largest messaging platforms in the world, and largely invisible. It essentially resides entirely in Apple’s messaging app, which is connected to and has its roots in SMS phone text messages. When you are connected to the internet and messaging an Apple user with iMessage turned on, you use the internet, you don’t incur any messaging fees, the messages you send are blue, and you get access to special iMessage features (audio messages, Animoji, etc.). If you are not connected to the internet, your message is sent as a traditional SMS, you may be charged fees by your carrier, the messages you send are green, and you don’t get access to the special iMessage features. If you’re in a text group, if even just one person doesn’t have iMessage, the whole group defaults to SMS.
There are good and bad things about being coupled to SMS. On the positive side, it allowed Apple to bootstrap iMessage usage, because iPhone users automatically got access to it when sending traditional text messages to other iPhone users. Being tied to SMS also means that users don’t have to think about what app to use–they just go to their Messages app to send a message, and it will be sent either by SMS or iMessage depending on the recipient.
Unfortunately, the negatives are also substantial. First, the seamlessness can be confusing in some situations. For example, my parents are currently visiting us in Beijing, and they don’t have any idea about the difference between iMessage and SMS, so they are sometimes confused about which messages are going to be free and which messages are going to incur international text charges. I suppose international travel is an edge case, but my assumption is that a majority of iMessage users are probably like my parents and are barely aware of the iMessage/SMS distinction.
Second, while SMS was hugely important when iMessage was launched, which was why the bootstrap effect was so powerful, in many countries today SMS has been largely supplanted by standalone messaging apps like WeChat, Line, WhatsApp, etc. My SMS usage, for example, is essentially limited to spam, some app logins, and communication with a limited number of US-based family and friends. At this point, tying iMessage to SMS limits iMessage’s potential for growth as a standalone messaging app/platform.
The biggest issue, perhaps, is iMessage’s function as a lock-in for iPhone users. If you leave the Apple ecosystem, you lose your blue bubbles, which may serve as a silly status symbol, but also, and probably more importantly, are required to participate in certain group chats with friends. This one factor is probably enough to keep iMessage limited to Apple’s platforms and tied to SMS, but it degrades the user experience and limits growth, particularly outside of the United States.