Been doing some work in MonoDroid and found that I needed to pass a user entered value from on Activity to another Activity in MonoDroid.  Here's how I did it.

In my sending Activity, I need to take some user user entered data and send it to my second activity.  Here is the code:

            string UserId = Convert.ToString(et.Text);
            if (!String.IsNullOrEmpty(UserId))
            {
                Intent i = new Intent();
                i.SetClass(this, typeof(CustomList));
                i.AddFlags(ActivityFlags.NewTask);
                i.PutExtra("TwitterId", UserId);
                StartActivity(i);
            }

 In this code, I have called .PutExtra and  passed it with a key.  In this case, I am passing a Twitter id.  In the code that is receiving the data, the code to retrieve the Twitter id is:

string twitterId = Intent.GetStringExtra("TwitterId"); 

The call to GetStringExtra() returns the value passed on the key.