It’s the “chat icon”. It is only available after you have done this: Open the settings dialog (the rightmost toolbar button), go to “Custom Settings”, and set the netplay_feature to 1 (one):
netplay_feature = 1
(In version 2.1.x, this setting has a graphical switch under Settings -> Experimental Features)
When you re-start FS-UAE Launcher, a new net play tab (a chat icon) is available.