I do not have figures with me on union membership, but those are easily available from sources in Colombia, from the Escuela Nacional Sindical, the Colombian version of the CLC.
I've seen reports of recent declines in unionism, but nothing substantial. It would seem to be cyclical. There have been no reports that I've seen of precipitous or unusual drops in union activity.
It's interesting to note, though, that there is a split in terms of support for the free trade agreement among unions in Colombia. Those that actually have a stake in the agreement, those from the private sector, are overwhelmingly in favour of the agreement. Those from the public sector, who don't have a stake in it and won't mostly be impacted by it, tend to be overwhelmingly against.