It depends upon the level of importance of the message to be communicated. As said by Mr.Mohan direct talk, it strengthens relationship bond. Face to face contact is always effective and can be easily understood. In direct talk feedback is instantaneous.
When the receiver and sender are far off, then we can go for letter but nowadays mails are used for this. Written communication like letter or mail whatever, serves as a record and is permanent for future reference.
If you are not able to manage your time but still wants to maintain relationship with your friends SMS fills the gap. Just even a forwarded message to your friends brings smiles to them as it meant that you are remembering that person every now and then in your busy schedule.
Just these are my views.