application.js

KEEPALIVE_INTERVAL = 5*1000;

Meteor.setInterval(function(){
    Meteor.call('keepalive');
}, KEEPALIVE_INTERVAL);

index.html

{{#each friends}}
    <li id="{{user_id}}" class="player-profile">
        
		<div class="friend-name">
        	{{#if online user_id}}
        	<img src="images/online.png" />
        	{{else}}
        	<img src="images/offline.png" />
        	{{/if}}
			<a data-toggle="modal" href="#profile-modal">{{name}}</a>
		</div>
    	
    </li>
{{/each}}

index.js

Template.index.online = function(user_id) {
    return Connections.findOne({user_id: user_id});
}

publications.js

CONNECTIONS_CLEANUP = 10*1000;

// this is server sided
Meteor.setInterval(function(){
    var now = +(new Date())/1000;
    Connections.find({last_seen: {$lt: now - 10}}).forEach(function(conn){        
        Connections.remove(conn['_id']);
    });
}, CONNECTIONS_CLEANUP);