The valuations are fine. Firstly, WordPress is not valued at anything as it is not a private company. You are thinking of Automattic, which is a managed host that owns wordpress.com and makes their money on ads and subscriptions. Their share of and profit from the WordPress ecosystem is much smaller than you think, blogs / websites are commodities at this point, and there is no reason to think there is a relationship between volume of content and valuation.

Meanwhile, Twitter and Reddit are massive social networks with millions of daily active users.