Axios:
Sources: Substack is raising $65M at a $650M valuation; a16z will lead the round  —  Substack is raising $65 million in new venture capital funding that would value the company at around $650 million, Axios has learned.  Existing investor Andreessen Horowitz is leading the round.

Source: http://www.techmeme.com/210329/p28#a210329p28