Quick reference: which platforms support timeline posts, long‑form content, sharing, commenting and DMs.
| Platform | Timeline Post | Serious Content | Share of Others' Post | Comment | DM |
|---|---|---|---|---|---|
| Facebook (Timeline) | Yes | Yes | Yes | Yes | Yes (Messenger) |
| Facebook Groups (others') | Depends on group rules | Possible | Depends on group rules | Yes | Not typical (Messenger used) |
| Yes (Tweets) | Yes (Threads / long posts) | Yes (Retweet / Quote) | Yes | Yes | |
| Substack | No typical timeline; publish posts | Yes (Essays / Newsletters) | Yes (via links in posts) | Yes | Limited / Not primary (reply to email goes to your inbox) |
| No timeline; posts in subreddits | Yes (depending on subreddit) | Yes (Cross-post or link) | Yes | Yes (Chat/PM) | |
| Medium | No timeline; publish articles | Yes (Long-form articles) | Yes (Linking) | Yes | No real DM |
| Status / Channel posts | Yes (Groups / Broadcast) | Forwarding possible | Yes | Yes (primary usage) |
Short answer: You cannot reliably ask readers to "DM" you on Substack or Medium, because neither platform provides a traditional, dependable in‑site direct‑message inbox.
Recommended alternatives: ask readers to reply to the Substack email (goes to your email inbox), or put a clear contact link on your Medium profile / article asking readers to reach you by email, website contact form, or your preferred social channel.