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Start with the channel where strangers already ask you questions in public, which for most small businesses is Instagram.
The reasoning is about where the repeated work is, not about which platform is best. Automation pays off where the same question arrives many times from people you do not know yet. On Instagram that is the comment section: price, availability, do you ship, is this still in stock. Each of those is a conversation you are having by hand right now.
Why Instagram first, concretely:
- Setup is one approval screen, with no business verification and no template review.
- There is no per message cost, so a post that does well does not produce a bill.
- The comment to DM loop is the single highest volume pattern in this whole category.
Why not WhatsApp first, even if that is where your regulars are: business verification and template approval take real days, conversations cost money, and your regulars are not the repeated stranger question. WhatsApp is the better second channel, once the Instagram flow is catching the volume and you want to keep those people.
Why not Facebook first: the Page flow is nearly identical to Instagram but usually lower volume for a small business, so it is a copy rather than a start.
Do one automation, the one that answers your most repeated question, and read the first fifty conversations before you build a second.
The Getting started article covers the order of setup for a first workspace.