If you live by SMS for contacting your friends but your SMS telco charges is getting too high, you might want to consider Twitter as a replacement. This is especially so if your friends is overseas, since most telcos charge a premium for sending messages internationally. And Twitter is basically free.
By default all messages you send on Twitter is public. But you can use Direct Message for sending to your friends. In order for Direct Message to work, you need to follow your friend, and your friend needs to follow you too.
Certainly you can use WhatsApp or messaging apps for such purpose. But Twitter is free and behaves quite similar to SMS.