May we keep our engagement secret from colleagues and social media?

Yes; what is sunnah to announce is the marriage, not the engagement. An engagement can still change. Keeping it within the family can protect both parties from speculation and embarrassment if it does not go ahead. But keeping it private is not the same as hiding it from the wali. Close family still need to know and be involved. If you want to share it, a brief announcement is enough, without photos of the two of you or personal details. Discuss with your fiancé and your families who should be told and when, so everyone is in agreement. After the akad, announce the marriage so your status is clear to the community. See Privacy in the household.