How do I implement a multi-line flexbox?

The flex box spec has changed in the last few months and you are using the old syntax. The new spec I believe is only currently implemented only in Chrome Canary and to some extent in latest chrome. (It's a little buggy) box-lines multiple is gone so to achieve this vertical layout there are a few ways. Using -webkit-flex-direction:column; so

<div id="flexbox">
    <div class="box">DIV 1</div>
    <div class="box">DIV 2</div>
    <div class="box">DIV 3</div>
</div>

and css :

#flexbox {
    display: -webkit-flex;
    -webkit-flex-direction: column;
    width: 500px;
    height: auto;
    min-height: 200px;
}

#flexbox .box {
    -webkit-flex: 1;

}

Using wrapping:

-webkit-flex-wrap: wrap; 

and setting your direction to:

-webkit-flex-direction: row;

So

<div id="flexbox">
    <div class="box">DIV 1</div>
    <div class="box">DIV 2</div>
    <div class="box">DIV 3</div>
    <div class="box">DIV 4</div>
    <div class="box bigger">DIV 5</div>
    <div class="box">DIV 6</div>
</div>

#flexbox {
    display: -webkit-flex;
    -webkit-flex-direction: row;
    -webkit-flex-wrap: wrap;
    width: 500px;
    height: auto;
    min-height: 200px;
    }

#flexbox .box {

    -webkit-flex: 130px 1;

    }

#flexbox .box.bigger {

    -webkit-flex: 330px 1;

    }

There's a whole bunch of examples on the spec page linked above.

Use the flex-direction attribute.

.container {
  flex-direction: row | row-reverse | column | column-reverse;
}

Refer to the following article: http://fend-tricks.com/flexbox-intro/