I once worked for a social network that was extremely popular in a particular geographic niche. It was even started before Facebook. And it was also written in

I once worked for a social network that was extremely popular in a particular geographic niche. It was even started before Facebook. And it was also written in PHP. And we rewrote in Ruby (not rails, at the time it was not mature in the ways we needed it to be). We made a lot of great technical achievements in doing this and our codebase became much much better. On a technical level I don't regret us doing that one bit. We went long on schedule and we made mistakes, but some of the best work of my career went into that and I'm immensely proud of it. But then in a matter of months of Facebook going to being open to people who weren't at a university or big organization (remember that?) our users, who got bored with our site not changing while we rebuilt all the tech, completely abandoned us and we went from profitable (having never taken any outside investment) to dead in another couple of years. This isn't to say that this wouldn't have happened to Facebook had they done something like this, but it is always a huge risk, even if you do everything right.