<div class="zen">
    <div class="zen-pts" data-chatter-widget="subscribeBtn" data-sfdc-entity-id="{!task.projectTask.Id}" data-sfdc-network-id="000000000000000" onclick="SfdcApp.Chatter.SubscribeButton.onClick(event);" data-uidsfdc="340">
        <span class="zen-media zen-mediaExt" data-chatter-subscribe="1" style="display: {!IF(task.isFollowed, 'none', 'inline')};">
            <a href="javascript:void(0);" class="zen-mrs zen-img">
                <img src="/s.gif" alt="Follow this task to receive updates in your feed." width="12" height="1" class="chatter-icon chatter-followIcon" style="margin-right: 0.25em;" title="Follow this task to receive updates in your feed." />
            </a>
            <span class="zen-mediaBody">
                <a href="javascript:void(0);" title="Follow this task to receive updates in your feed.">
                    Follow
                </a>
            </span>
        </span>
        <span class="zen-media zen-mediaExt"
              onblur="SfdcApp.Chatter.SubscribeButton.onHover(event, false)"
              onfocus="SfdcApp.Chatter.SubscribeButton.onHover(event, true)"
              onmouseout="SfdcApp.Chatter.SubscribeButton.onHover(event, false)"
              onmouseover="SfdcApp.Chatter.SubscribeButton.onHover(event, true)"
              data-uidsfdc="341"
              style="display: {!IF(task.isFollowed, 'inline', 'none')};">

            <img src="/s.gif"
                 alt="Stop following this task to stop receiving updates in your feed."
                 width="12"
                 height="1"
                 class="chatter-icon zen-mrs chatter-checkedIcon zen-img"
                 style="margin-right: 0.25em;"
                 title="Stop following this task to stop receiving updates in your feed." />

            <span class="zen-mediaBody">
                Following
                <a href="javascript:void(0);" class="zen-mls zen-imgExt" data-chatter-subscribe="0" title="Stop following this task to stop receiving updates in your feed.">
                    <img src="/s.gif"
                         alt="Stop following this task to stop receiving updates in your feed."
                         width="12"
                         height="1"
                         class="chatter-icon chatter-unfollowIcon"
                         style="margin-right: 0.25em;"
                         title="Stop following this task to stop receiving updates in your feed." />
                </a>
            </span>
        </span>
    </div>
</div>

function unfollowHandler(item){
  Visualforce.remoting.Manager.invokeAction('{!$RemoteAction.ctrlMetro.unFollowItem}', item.id, userId, unfollowReturnHandler);
return false;
}

function followHandler(item){
    Visualforce.remoting.Manager.invokeAction('{!$RemoteAction.ctrlMetro.followItem}', item.id, userId, followReturnHandler);
    return false;
}

<img id='imgFollow' onClick="followHandler(this); return false;"  title="click to follow" style="height:16px; width:16px; display:none;" src="{!URLFOR($Resource.StatusIcons,'error_icon.png')}"/>
<img id='imgUnfollow' onClick="unfollowHandler(this); return false;"  title="Following: click to unfollow" style="height:16px; width:16px; display:none;" src="{!URLFOR($Resource.StatusIcons,'check_icon.png')}"/>

@RemoteAction
    global static string unFollowItem(string itemId, string userId){
        try{

        EntitySubscription es = [select id from EntitySubscription where parentId=:itemId and subscriberId=:userId limit 1000];
        delete es;
        return itemId;
        }catch(System.Exception ex){
            system.debug(ex);
            return 'error';
        }

        return 'error';
    }

    @RemoteAction
    global static string followItem(string itemId, string userId){
        try{
        EntitySubscription es = new EntitySubscription(parentId=itemId, SubscriberId=userId);
        insert es;
        return itemId;
        }catch(System.Exception ex){
            system.debug(ex);
            return 'error';
        }

        return 'error';
    }