1. Before creating a .Net MVC application we have to register domain name that will be used for web site at the Facebook development site http://developers.facebook.com/setup/. After successful registration we will have facebook APIKey and facebook Secret.

2. Now let's create a simple ASP.NET MVC application in VS:

I will use facebook api button in this sample to show alternative log in option to the user.
Let's change _LogOnPartial.cshtml file in such a way:

@if(Request.IsAuthenticated) {
    <text>Welcome <strong>@User.Identity.Name</strong>!
    [ @Html.ActionLink("Log Off", "LogOff", "Account") ]</text>
}
else {
    <fb:login-button perms="email,user_checkins" onlogin="afterFacebookConnect();" autologoutlink="false" ></fb:login-button>
    <div id="fb-root" style="display:inline; margin-left:20px;"></div>
    @:[ @Html.ActionLink("Log On", "LogOn", "Account") ]
}
<script language="javascript" type="text/javascript">
    window.fbAsyncInit = function () {
        FB.init({ appId: ' -- YOUR REAL APPLICATION ID SHOUD BE HERE --', status: true, cookie: false, xfbml: true });
    };
    function afterFacebookConnect() {
        FB.getLoginStatus(function (response) {
            if (response.session) {
                window.location = "../Account/FacebookLogin?token=" + response.session.access_token;
            } else {
                // user clicked Cancel
            }
        });
    };
    $(document).ready(function () {
        if (document.getElementById('fb-root') != undefined) {
            var e = document.createElement('script');
            e.type = 'text/javascript';
            e.src = document.location.protocol + '//connect.facebook.net/en_US/all.js';
            e.async = true;
            document.getElementById('fb-root').appendChild(e);
        }
    });
</script> 

Following elements were added to the control:

• facebook login button (fb:login-button) here,
• container which will contain all facebook scripts (div id="fb-root")
• FB initialization script (FB.fbAsyncInit). You HAVE TO replace sample appId value with real one, received when registering your app on the facebook development site.
• afterFacebookConnect - script which will be called after user closed FB login dialog window, (after successful or failed login)
• script for loading FB javascript libraries (e.src = document.location.protocol + '//connect.facebook.net/en_US/all.js';)

After successful login we will have access_token value, and now we can load detailed user's info, store this info (if we need to) and authenticate the user. To do this we will redirect user to the Account.FacebookLogin action, and pass access_token value as a parameter to this action. So at this stage we will implement "FacebookLogin" action.
Created action will look like this: 

using System.Web.Mvc;
using System.Web.Security;
using MVCOAuth.Models;
using System.Net;
using Newtonsoft.Json.Linq;
using System;

namespace MVCOAuth.Controllers
{
    public class AccountController : Controller
    {
        [HttpGet]
        public ActionResult FacebookLogin(string token)
        {
            WebClient client = new WebClient();
            string JsonResult = client.DownloadString(string.Concat("https://graph.facebook.com/me?access_token=", token));
            // Json.Net is really helpful if you have to deal with Json from .Net http://json.codeplex.com/
            JObject jsonUserInfo = JObject.Parse(JsonResult);
            // you can get more user's info here. Please refer to: http://developers.facebook.com/docs/reference/api/user/
            string username = jsonUserInfo.Value<string>("username");
            string email = jsonUserInfo.Value<string>("email");
            string locale = jsonUserInfo.Value<string>("locale");
            int facebook_userID = jsonUserInfo.Value<int>("id");

            // store user's information here...
            FormsAuthentication.SetAuthCookie(username, true);
            return RedirectToAction("Index", "Home");
        }

 And that's it! We have integrated alternative facebook authentication on the MVC site.
Before login:

After successful facebook authentication:

Hope this will be helpful for someone! 

Sam