Back to the X3, considering it's an express bus service between two major cities and having operated for well over a year now, I'd say passenger growth has been pretty disappointing. The timetable was scaled back considerably last month with the withdrawal of a few early morning and evening journeys. I still see completely empty buses at certain times of the day too. Off-peak it tends to attract the free-travelling concessionaires especially those who don't pay for a Metro Gold Card. GNE aren't fully reimbursed for carrying these passengers so these journeys could possibly be operating at a loss or barely breaking even.
Having regularly been travelling on the metro over the past couple of months, trends seem to crop up. Seaburn is a very popular station - according to Wikipedia it's the busiest station on the Sunderland extension after Sunderland and Park Lane. East Boldon and Stadium of Light are also very well used stations, both of these having park and ride facilities. All three stations are very short walking distances from the X3 route.
I agree it would be a huge shame if the X3 was withdrawn, but it's nothing definite yet, it's just someone's post on Facebook so can't be assumed as fact!

Personally, I think I'd be happy to see the return of daytime 26's to Heworth, but only if the two morning trips started from Sunderland/Southwick instead of Hylton Retail Park! That would mean I could get to work using one bus and one metro instead of one bus and two metros.