Mastodon is the number 1 network with over 4.000.000 users followed by Diaspora with over 600.000 members. On place 3 comes #Pleroma with over 30.000 members followed by #Hubzilla on place 4 with over 10.000 members and on place 5 comes #Friendica with almost 4.000 members.
According to this calculation it would be clear that #Mastodon is the clear winner but here the idea of the #Federation (Association) takes hold. Let's see who can communicate with whom and what the result is, start with the winner.
Mastodon can communicate with Pleroma, Hubzilla and Friendica so that results in 4.044.00 users.
Diaspora can communicate with Hubzilla and Friendica, resulting in a total of 614,000 users.
Pleroma can communicate with Mastodon, Hubzilla and Friendica and has a comparable reach to Mastodon of 4,044,000 members.
Hubzilla can communicate with Friendica and Pleroma, resulting in a reach of 44,000 members.
Friendica can communicate with Mastodon, Diaspora, Pleroma, and Hubzilla, resulting in a reach of 4,644,000 people.
Friendica is the clear winner in terms of reach. Now everyone just has to find out for themselves which platform suits them best.