The term "Text Neck" has been coined to describe the posture formed by leaning forward for prolonged periods.
We now spend so much time on smart phones to socialize or to get work done that we don't realize the stress we are placing on our body.
It is not just smart phone use we have to be careful of either. It is any activity, which puts us in this position (below). Another example is reading.
On average an adult head can weigh ~10lbs. When the neck is bent the relative weight is magnified, therefore creating a significant strain on the neck and spine. This in turn leads to potential neck strain and muscular dysfunction and even nerve damage
Tips to protect your neck and spine
Hold your smartphone up and at eye-level while using it
Lay on your back and bring the phone to eye-level while using it
Take breaks
Stretch daily: especially the anterior deltoids (front of shoulders)
Strengthen posterior deltoids (back of shoulders) - especially so if you're already accustomed to strengthening the anterior deltoids
Pay attention to your posture