The only thing my inexperienced mind could follow was: 

• attributeList is a literal control in default.aspx that is used to set html attributes according to PortalSettings.ActiveTab.SkinDoctype in the SetSkinDoctype() method on Init stage.
• SetSkinDoctype() is using HtmlAttributes.Add("lang", strLang) function;

The weird thing is that I use the same function in PreRender stage as SetSkinDoctype does, which is being called after the Init stage in default.aspx, but still can't get it working...

Yeah, I must've been a bit slow in reading the right ideas, but eventually got it working through the attributeList literal control! 

    Protected Sub Page_PreRender(sender As Object, e As System.EventArgs) Handles Me.PreRender

            Dim defaultPage As CDefault = TryCast(Me.Page, CDefault)
            Dim AttributeList As Literal = TryCast(defaultPage.FindControl("attributeList"), Literal)
            AttributeList.Text &= " xmlns:og=" & Chr(34) & "http://opengraphprotocol.org/schema/" & Chr(34)
            AttributeList.Text &= " xmlns:fb=" & Chr(34) & "http://www.facebook.com/2008/fbml" & Chr(34)

    End Sub

Thanks a lot, Cathal!

P.S. Although this problem solved, still it is not a best solution, since it depends on the literal control in default.aspx... If one day the core DNN team decides to remove (the literal control) and implement some other mechanism for html head tagging, the approach won't work.