I might be wrong, but .... for a socket to be defined as being a 'USB socket' it needs to conform to certain standards ... physical size, pin connections, and power supply, etc.
I'm sure that the Finnair USB socket will be able to supply enough power to charge your iphone. In 2012 I flew with Emirates to/from the far east via Dubai, and 3 of the 4 flights were equipped with USB connectors in the seat-back screen system. I used the socket to re-charge my MP3 player and my phone, and to keep my phone charged while my GPS tracker was running.