Poke (or alternately tap, nudge, hi, hello, ping, wink, flirt, pat, wave, etc.) is a feature on a few silos and messaging networks that's a silly method of attempting to get someone's attention, annoy them, or say "I'm thinking about you", typically in an attempt to start an actual conversation. It's an online version or equivalent of tapping someone on the shoulder in real life.
A close equivalent with a broader range of semantics is available using reacji, which could be used to effectuate a similar intent, though more often those are used in response to another post or event online.
- Just for fun Chris Aldrich poked David Shanske via webmention at Poking David Shanske using a 👉 emoji wrapped with a link to David's site and a u-in-reply-to class as well as highly experimental (read: non-existent other than these) u-tap-of and u-poke-of microformats. He sincerely hopes no one follows his lead, particularly as while he was doing a minimal amount of research, he coincidentally discovered that Facebook is experimenting with expanding this feature on their service with a broader form of "Greetings".
- gRegor Morrill then poked Chris Aldrich, but definitely not because he read the above as a challenge. https://gregorlove.com/2017/12/poked-chris-aldrich/
Poke was an original feature of the Facebook platform that was never removed. "When we created the poke, we thought it would be cool to have a feature without any specific purpose," explains Facebook. "People interpret the poke in many different ways, and we encourage you to come up with your own meanings." 
- Some used the poke functionality to prompt their friends to return to the Facebook platform to update their current status if they hadn't in a while.
- How to poke someone on Facebook.
- Poke page
- Applications like SuperPoke! (now defunct) allowed users to indicate more differentiated semantics.
- 2017-12: Facebook is Testing Greetings Buttons That are Like Pokes on Steroids
Since poking is basically the process of posting a message to a friend’s wall (or site) to say hello, flirt or even annoy them – why couldn’t you do the same thing with a simple message?
There has generally been no real accepted etiquette for these posts, and people have tended to make up their own rules. As a result some find these annoying.