Something like that. Facebook saw that Instagram was adding users fast and dominating mobile (a weakness of Facebook's). The plurality, if not the majority, of Facebook activity revolves around photo sharing and photo interactions (photos get way more engagement than the sharing of links).
So Zuck decided to take them out of the competitive landscape now, while it could still afford to, rather than risk Systrom turning Instagram into a major social network that siphoned away significant amounts of activity from Facebook.com