https://www.youtube.com/watch?v=oAatPPEaZDA
Specifically, online communities and communication tools tend to rot over time. Remember Paltalk? AOL Instant Messenger? Microsoft Messenger? ICQ? Skype? Lynq? Myspace? IRC? CU Seeme? WebEx?
Their replacements didn't win by being technologically better (you have to work hard to tell the difference between Facebook Messenger and AOL Instant Messenger) but because they deteriorated and people could get a fresh start elsewhere.
Very often new products launch using the incumbent's network as a launch pad. If you were starting, say, a football discussion site, you can pull people off Facebook/Reddit football groups, or some sports channel forum.