Edit in JSFiddle

var TextCounter = React.createClass({
  propTypes: {
    text: React.PropTypes.string
  },

  getInitialState: function() {
    return {
      text: this.props.text
    }
  },

  _textChange: function(ev) {
    this.setState({
      text: ev.target.value
    });
  },

  render: function() {
    return React.DOM.div(
      null,
      React.DOM.textarea(
        {
          rows: 5,
          cols: 30,
          value: this.state.text,
          onChange: this._textChange
        }
      ),
      React.DOM.p(
        null,
        React.DOM.small(null, 'Count: ' + this.state.text.length)
      )
    )
  }
});

React.render(
  React.createElement(TextCounter, {
    text: 'Type something'
  }),
  document.getElementById('text-counter')
);

<div id="text-counter"></div>