Off course i know, we can invite freinds to join dropbox and get the free space. But most of the people use dropbox in nowadays. Do you think this is the best way to get the free space? According to processing, the friend who you invite, must don't use the dropbox before. So, you must invite the friend, who have a email, which email don't have dropbox account, and you use this email to create a new dropbox account, then you can get the free space. So, this is not a problem and good solution to promote more peoples to use dropbox, this is only romote email user to use dropbox. Example, i got 10 email, so i can create 10 dropbox account, but actually just 1 person who use dropbox.
Google also makes all of its money off of ads and the sale of information, so it's able to subsidize its cloud offerings. Dropbox has no other source of income. It doesn't sell ads (Google), it doesn't make hardware (Apple) and it doesn't sell any other software or service (Microsoft). All of these other companies that offer cloud storage make their money elsewhere, and use the cloud services as a way to get people in and exposed to those other services. All Dropbox has is its cloud storage. There's no other product or service that offsets the loss from giving away all that space.