And also work to make yourself someone who is kept on when there are layoffs. This means focusing more on your work than on your social life. Since she apparently did you a favor back in August and has seniority over you, I would think that none of these:
…are appropriate. If you take the bitterness and nastiness out of numbers 2 and 3 you might have a decent plan: deal with her in a professional manner, treating her with friendly courtesy, as you do every other coworker. It is up to YOU to regulate your emotions. It is up to YOU to avoid silly drama games, and if you can’t handle social media or non-work-related group communication, it is up to YOU to remove yourself without making a public fuss.
These are ultimately your issues, and they are about your boundaries, not about her behavior or your romantic feelings. Professionals don’t make announcements to others about the interpersonal conditions under which they can maintain emotional stability; they create these conditions for themselves internally. An exception would be clear abuse of power, e.g. sexual harassment, but that’s not at play here from the sounds of it.
Professional. Courteous. Hard-working. Friendly without seeking to become friends. A go-getter. These are the qualities that make you a good guy to have on the team. Ultimatums, flounces, angry body language, and gossip are going to maintain you forever in a place of professional irrelevance, because they smack of immaturity. Your three options suggest you are still very much emotionally invested in this situation. Stop it.