Well, I want to know how to interact with the other user, i.e., can I add someone, send messages or other social network service?
No - the text of this question puts it well:
... the focused Q & A, low-discussion framework of SE means that although people are users of the system, structurally the site is not set up to directly create relationships between users, but only between questions and answers. For example, the software does not provide a facility for users to send each other messages.
However, you can "ping" someone in a comment using an "@-reply".