I was frustrated by select2 plugin. I did not understand why on plugin’s homepage placeholder works and on my site doesn’t. I tried to google it and i found that you must add
<option></option> for your select. I tried to add multiple options with value but placeholder did not work. The first option must be empty in order to get placeholder working!
<select id="sel" placeholder="Hello">
Here is working example: http://hromnik.com/web/select2/
Check source code for details.