My lecturer said that since hard disk space is so cheap these days, most organisations do not delete the data on the disk. Rather, they only logically delete it (remove the link to the data). If this is the case, would it also mean that organisations like Facebook still have the data of users who delete their pictures, comments etc.? Is it really that troublesome to physically remove data?
It is not troublesome, removing these "links" have been done for a very long time, even when disk space was expensive. Since CPU cycles were also expensive, there was no point in securely "overwriting" (like shred) data. If you remove the logical file table entries, the space it addressed as empty and was just overwritten.
Alas, it is not "troublesome", to remove data - it is just wasted time / CPU cycles, and then we're not talking about the ability to recover this data.
Companies such as facebook store their data over several media, not just a hard drive. First off, it is located in multiple servers from an availability / performance point of view, and is backed up several times. Plus, their Terms of Service stipulates that it is still somehow accessible and could be "used" (though within limits) even after your account is removed.