Those aren't the final numbers as it doesn't take account of those without pre-booked seats that haven't checked in. Nonetheless, you wouldn't imagine final numbers to be particularly respectable.
I'm not sure this is a viable option for city-city in terms of time as the door to door train journey would surely be quicker, though with next-day return fares of £60 to £100 it is a cost effective option. I would doubt though that many people would even think to look at flights.
It is a good option if you need to travel from Southend's immediate local catchment (say, Essex/Thurrock/Barking etc) to Manchester or the north-west and for a weekend city break, but I just can't see enough demand. Neither do I think enough people connect through MAN to elsewhere.
I see another 1000 tickets are being given away. You have to wonder how many of those that have travelled to date have been on freebies.