How to design a relationship database table to store friendship relationship?
I'd create a table a lot like the one that you have.  I'm using SQL Server data types and syntax, you may need to tweak depending on your platform.

CREATE TABLE FriendStatus
(FriendStatusId BIGINT PRIMARY KEY IDENTITY(1,1),
FromUserId BIGINT,
ToUserId BIGINT,
StatusId TINYINT,
SentTime DATETIME2,
ResponseTime DATETIME2);


Indexing of the table will be critical as the table grows to the tens and hundreds of millions.