I still don't understand how people can say Twitter and Facebook is dead or dying. Your outlook on the world of SEO and advertising must be 0.
Facebook still has over 2 billion active users. 22% of the entire world is on Facebook. Yes, anyone under 24 doesn't use Facebook as much, but there are different platforms to capture their attention.
Twitter is one of the best platforms to capture people in the height of their emotion. It is an amazing tool for customer service and to capture people in the heat of needing or wanting something.
As for tweets, i used to work for a platform which developed an API integration to find keyword tweets and auto respond. Leave at least a minute between tweets, make it seem natural and you should be fine. I probably wouldn't exceed around 40 a day to start with though.
If you have an approved blue tick account, you can post as much as you want.