It looks like you need to build omni-channel to deliver message about one particular (past?) event to your audience though multiply ways. As I understand we are not talking about new sales (direct sales - tickets for new events) but about something like "smart follow ups" after spent events to retain visitors (cross-sales) and attract new ones (up-sales)
I agree (and upvote! :) ) that chat-bots are really helpful in the broad sense: FB messenger, Viber, WhatsApp, Telegram, Line, WeChat & etc. It depends on covered area/audience. Anyway, it is just a tool.
The real value is the useful content that may be shared through two groups of people: ones of them were attendee (closed group) and last ones weren't (wide, open group). What is it? Slides? Videos? Insights? Emotions? Something that motivates people to attend events (and buy tickets) more and more.
So, it is about strategy, not about tools.
Chatbots would be the best option.