"We already have a huge user base, we'll just automatically sign them up for <new thing> and get it off to a running start". No, these users will hate you for it and in fact ruin the experience for the select few who are actually interested. Meanwhile executives will tout the rosy user growth metrics, declare the launch a success and collect their bonuses.

There is no substitute for old-fashioned organic growth. Start small. Focus on early power users and listen to their feedback. Have something unique that sets you apart from the competition and attracts more users. This is how Instagram, Snapchat, TikTok and the like were able to establish themselves in Facebook's presence while Google+ failed.