Backing up online has two issues: 1) cost. Dropbox does not offer very high storage solutions so far. I have 3TB of videos and photos, where am I going to store it ? 2) access speed and upload speed. Especially upload. It takes forever to upload, unless maybe you have fiber connection...
The technical solution for most people is just not there yet. It's way cheaper to backup on a solid drive and store it at your friend's.
There are a few online backup services which let you mail them a disk if uploading would take too long. A quick bit of googling found Mosaic Archive (http://www.mosaicarchive.com/online-photo-backup/), for example. I haven't used them & don't know anything about them, but their website says they provide this service. I'm sure there are others too.
The technical solution for most people is just not there yet. It's way cheaper to backup on a solid drive and store it at your friend's.