Best Practise of Making notifications for logged in user
<p>I am making a web-site , which is a social network. 
There is need of making overall notification block/bar on the website. </p>

<p>User should be notified when 
a) there is new message
b) there is new friend request
c) there is new photo rating 
....</p>

<p>As application (php/mysql) should be made with future calculation of minimum 100-1500 users online, what is the best practise to make such notificaion display? 
Should I make checks with each time users refresh their browser, or Jquery script which will ping the special check.php script each 10-15-20min...??? What is most professional practise ? Who can help? </p>
