Jefferies, Cedar Rock Capital and SKAGEN are the latest to join Plato Partnership as founder members.
Former banker admitted sharing information on clients with friends via messaging app.
Neptune has 17 bond dealers live on its platform, with four more expected to go live in the first quarter this year.
Jefferies saw significant gains year-on-year in its equities and fixed income businesses.
Jefferies rescued KCG in 2012, but the bank confirms it is no longer majority shareholder.