- Reddit didn't do anything to Usenet. Reddit postdates Usenet's glory days by over a decade.
- Sears, a dying company, bought K-Mart, and they died together. K-Mart didn't do anything to Sears.
- Walmart didn't do anything to K-Mart except sell stuff cheaper and work on its supply chain. Wikipedia article says this: "Unlike its competitors Walmart and Target, Kmart failed to invest in computer technology to manage its supply chain."
I would argue that Facebook had a much bigger impact pulling users there.
As did multiple independent web forums (based on phpBB etc.) which allowed closed communities and moderation and we're more approachable for beginners.