SmugMug and Flickr are wonderful for sharing photos, but I don't use them for backup for two reasons. 1) They do not support RAW and 2) unlike dedicated backup services, restoring photos to a local hard drive is painful because they don't provide bi-directional syncing.
Google Photos has amazing software for search and organization, but it compresses your photos to 16 megapixels.
Like @Chris, I use Google Drive because I shoot and store all my photos in RAW. I found Google Drive to be the cheapest lossless way to store and access RAW photos in the cloud. Dropbox is great too, but if you have terabytes like me, Google Drive is cheaper. I have an enterprise Google Drive account where I pay $40/month for 10TB.
Like @petebocken says, "remember that you shouldn’t trust any of these services as a true single backup." Losing photos in the cloud does happen, like in the case of PictureLife. Or hypothetically, what if your billing lapses while you're on an extended vacation, and Google deletes your account?
For this reasons, I have a Synology NAS at home that stores a complete local copy of my photo library. The Synology runs a Google Drive assistant that manages syncing and archiving the photo library.