It's easy to not delete and stop using to serve your purpose. I loathe facebook et al., but some employers (fewer and fewer these days) want to see my online presence. To not have a facebook was considered strange, so for that reason, I keep it around. However, as far as maintaining an identity, or holding on to your custom facebook url, that’s a non-issue. Delete away, and create again—facebook is intelligent enough to reconnect you to all those people if need be. Personal identity is strengthened by other's endorsement. That's easy to get again with FB, especially if you're who you really are. It's difficult to take someone’s identity in that sense and it be valuable to the person doing it.

> some employers (fewer and fewer these days) want to see my online presence.

That's a really huge red flag. I don't think that I'd be willing to risk taking a job with a company that asked for this.