The wording I used on my own post was :
I just supported the @BookCrossing site by purchasing "Wings" membership. The Wings program lets you show your love and support for #BookCrossing & allows you to flutter for a year from the purchase date. Get yours here: https://www.bookcrossing.com/store#!/Wings/c/554365/offset=0&sort=norma
....and here is the link to it my Tweet if that helps:
Just thought it might be a good idea for drumming up some much needed memberships.