Okay, it's time to tell new users a secret about Mastodon.
Mastodon is (whispers) part of a much bigger network 😮
Mastodon servers use an open standard called ActivityPub to talk to each other. That's how you interact with people on other Mastodon servers.
But, dozens of other federated networks also use ActivityPub, so (this is the good bit!) you can follow people on these other networks from your Mastodon account!
For example:
PixelFed (https://pixelfed.org) is a photo sharing network, here's a random example account @Iancylkowski
PeerTube (https://joinpeertube.org) is a video sharing network, with a p2p system that allows videos to go viral even on small servers, here's a random account @craftykat
BookWyrm (https://joinbookwyrm.com) is a social reading site, an open alternative to Amazon's Goodreads, here's a random account @mouse
Together, Mastodon and these other services form... The Fediverse 
(whispers) Click on the link...
https://framatube.org/w/4294a720-f263-4ea4-9392-cf9cea4d5277
A question though. My pixelfed posts are not visible on mastodon. I have found and followed my pixelfed profile, and it’s empty when viewed from mastodon. When I look at my profile directly on pixelfed, the posts are there. What am I doing wrong?
You're not doing anything wrong.
Is the PixelFed account quite new?
If no one on your Mastodon server has followed you PixelFed account, then your posts won't be visible there.
However, after at least one person on a server follows your PixelFed account, all new posts from the account will be visible on that Mastodon server.
So, basically, as you get more followers your posts will become more and more visible across the Fediverse (including Mastodon).
@feditips @michalzadkowski my mastodon and pixelfed accounts are coming up for 18 months, so not exactly new and I have the same problems. In fact many accounts I follow are incomplete and frequently display a link at the top advising you to access the account on the original server. If you click on it you can see all their posts but you cannot interact with them as you are not logged in to an account on their server. Is there a way to rectify this?
@feditips @michalzadkowski I see, but as my mastodon and pixelfed accounts follow each other wouldn't this already be the case?
It would only see posts made after someone on your server followed the account. Any older posts would be ignored, unless someone on your server sees them shared or searches for them.
@feditips @michalzadkowski It doesn't seem to be the case, but thanks for your help
@hackbyte @feditips @michalzadkowski From my mastodon account I can see nearly all my pixelfed posts, over 150 of them. The most recent ones are missing. From my pixelfed account I can see only 2 random posts from my mastodon from last year.
@michael @hackbyte @michalzadkowski
If there are posts missing, try searching for their URLs and see if that has an effect?
@michael @michalzadkowski
If you're following an account on another server, your server will only start "seeing" new posts made after someone on your server followed that account. So, anything the remote account posted before they were followed from your server will not be visible from your server.
BUT... you can force your server to "see" older posts too by copying and pasting their URL and searching for that URL in the search box on your server. If you then refresh the account's profile that was missing items, you will see the older post there too. (You would need to search for each post separately if you want them all to be visible on your server.)
Every time you search for an old post, it gets added to what the server can see.
You can also use the search trick to force your server to see accounts that no one on your server follows yet.