There is a big difference between platforms where it is difficult to get your data out and platfoms where it is impossible to get your member's data out.
This matters. If you move platforms, you don't want members to register again. You don't want them to lose their post counts and all record of their contributions.
You also want to backup your community data on a periodic basis, unless you risk the black swan event of your platform provider losing everything.
Some people draw the line between platforms where exporting data is easy and those where exporting data is difficult. They reject a lot of great platforms this way. They should be drawing the line between difficult and impossible.
Facebook pages is impossible. You can't export the data of members. You can't get their names, e-mail addresses and import them into another platform.
Ning is difficult. You can get the data out, but you will probably need a decent coder/developer to do it. Likewise, many forums can be difficult, but not impossible.
If your community is strong and you're paying to move to a different platform, a small fee to someone to transfer data from one database to another probably is good value.
Don't reject the platforms that are difficult to extract data from. Reject the platforms that are impossible to extract your data from.