Right. Easiest maybe, certainly not only.
What? The core aspect of FB/Twitter/etc. is twofold
1. No guaranteed-attention obligation from your circle, so you're free to share more things
2. Algorithms for YOUR attention so you see what you want to see in the order of importance. If you have very little time, you only see the most important things.
Email, Slack etc. are not even in the same category, it's like comparing a postcard to a newspaper headline
Not sure if I've ever believe it was the only way for them to grow. I hate Facebook (product) for what it has become but it is an excellent tool for its original use, which is keeping in touch with friends/family/communities.