Working Chatter in Apex. http://deferloader.blog.uhuru.co.jp/?p=3896

ChatterInApex.page

<apex:page controller="ChatterInApexController">
    <apex:form >
        <apex:selectList value="{!communityId}" size="1">
            <apex:selectOptions value="{!communityOptions}"/>
            <apex:actionSupport event="onchange" rerender="feed"/>
        </apex:selectList>
        <apex:commandButton value="Post" action="{!postFeed}" />
    </apex:form>
    
    <apex:outputPanel id="feed">
        <apex:repeat value="{!newsFeedItems}" var="feedItem">
          <div>
            <apex:image style="margin:4px" width="25" url="{!feedItem.photoUrl}"/><br/>
            User: <b>{!feedItem.actor.name}</b><br/>
            Text: <b>{!feedItem.body.text}</b><br/>
            Likes: <b>{!feedItem.likes.total} [
                <apex:repeat value="{!feedItem.likes.likes}" var="like">
	                {!like.user.lastName} {!like.user.FirstName}, 
	            </apex:repeat>
	  		] </b><br/>
            
            <apex:outputPanel >
              <apex:repeat value="{!feedItem.comments.comments}" var="comment">
                 <div style="margin-left:25px">
                   <apex:image style="margin:4px" width="25" url="{!comment.user.photo.smallPhotoUrl}"/><br/>
                   User: <b>{!comment.user.name}</b><br/>
                   Text: <b>{!comment.body.text}</b><br/>
                   Likes: <b>{!comment.likes.total} [
			           <apex:repeat value="{!comment.likes.likes}" var="like">
			               {!like.user.lastName} {!like.user.FirstName}, 
			           </apex:repeat>
				   ]</b>
                 </div>
              </apex:repeat>
            </apex:outputPanel>
          </div>
        </apex:repeat>
    </apex:outputPanel>
</apex:page>

ChatterInApexController

public class ChatterInApexController{
    
    public String communityId {
        get;
        
        set {
            if (value == '') {
                communityId = null;
            } else {
                communityId = value;
            }
        }
    }
    
    public static List<SelectOption> getCommunityOptions() {
        List<SelectOption> options = new List<SelectOption>();
        
        options.add(new SelectOption('', 'Internal'));
        
        ConnectApi.CommunityPage communityPage = ConnectApi.Communities.getCommunities(ConnectApi.CommunityStatus.Live);
        for (ConnectApi.Community community : communityPage.communities) {
            options.add(new SelectOption(community.id, community.name));
        }
        return options;
    }
    
    public List<ConnectApi.FeedItem> getNewsFeedItems() {
        return ConnectApi.ChatterFeeds.getFeedItemsFromFeed(communityId, ConnectApi.FeedType.News, 'me').items;
    }
    
    public PageReference postFeed() {
        ConnectApi.MessageBodyInput messageInput = new ConnectApi.MessageBodyInput();
        messageInput.messageSegments = new List<ConnectApi.MessageSegmentInput>();
        ConnectApi.TextSegmentInput textSegment = new ConnectApi.TextSegmentInput();
        ConnectApi.MentionSegmentInput mentionSegment = new ConnectApi.MentionSegmentInput();
        ConnectApi.FeedItemInput input = new ConnectApi.FeedItemInput();
        
        mentionSegment.id = 'XXXXXXXXXXXXXXX';
        messageInput.messageSegments.add(mentionSegment);
        
        textSegment.text = 'test';
        messageInput.messageSegments.add(textSegment);
        
        input.body = messageInput;
        ConnectApi.ChatterFeeds.postFeedItem(communityId, ConnectApi.FeedType.News, 'me', Input, null);
        return null;
    }
    
    public PageReference choose() {
 	 	return null;
 	}
}