A long-winded answer to....
Question: what are my anecdotal thoughts the cause of Facebook's growth?
"Facebook has always focused more on user experience than anything else. MySpace became riddled with spam and cluttered with ads so much so as to obscure the user experience and users were leaving MySpace in drove around ‘06/’07 to go to the cleaner, more user-friendly FB experience. They also rapidly expanded and marketed to international markets—namely Scandinavia initially (right when Swedish invasion was hitting)
But the single most important factor to Facebook's adoption? The Newsfeed. Streams. The network was more useful because it captures and reports back to users all of the actions taking place within their network. Solid gold utility.
Also, Facebook was first to release a developer API and we saw the rise of the infamous Facebook apps, which drove adoption of the platform.
Because of Facebook’s focus on authentic user experience (most users a registered under their real names as early membership required a university email address) it became a “safe zone” for an older, more professional demographic as well....
Then there was the release of the free Facebook “Business Page” which allows brands to integrate in a non-threatening, non-spammy way into the platform. There’s been a rise in demand from brands to establish these Business Pages as part of their digital marketing mix, driving adoption
Basically, Facebook has focused on offering a better user experience (some even go so far as to attribute the rapid adoption to one single, useful feature-- the News Feed feature), and their growth is a testament to just how vitally important putting user needs and experience first is to any platform.
MySpace allows for branded profiles, but at a cost of $150k
On a side note, MySpace has been late in innovation—they focused more on selling every square inch of space with ads they failed to take into account the poor user experience (spam, the constant “technical difficulty” warning, poor navigation and ugly UI), they have since made strides to change this with a recent re-design and developer API. I think it came a little late, and it will be interesting to see their next steps. I know Chris De Wolfe has said he wants to turn MySpace into a portal site much like a new MSN.com...."