A little over a year ago, I got an email from a college friend, telling me about her boyfriend’s startup. He and some friends had a made an app that let you take pictures, apply filters to them, and share them with friends. “That’s kind of boring,” I thought. I decided not to write about it.

On Monday, my friend’s boyfriend’s startup was acquired for a cool billion dollars, by Facebook.

Perhaps, then, I’m the last person on earth who should be predicting who will be the next Instagram.

Or perhaps, chastened by my mistake, I’m just the right person to do so.

Either way, here are a couple of rising social startups that Mark Zuckerberg might want to throw his buckets of IPO money at.

1. Pinterest

This is the most obvious one. You can hardly load a webpage today without hearing something about Pinterest, the social pinboarding site that recently became the third-most-popular social network, after Facebook and Twitter. On Wednesday, in fact, Amazon and eBay added Pinterest buttons to their sites. How Facebook-like! In February, TR’s own Rachel Metz wrote a fun review of Pinterest (“I immediately let my inner magpie fly”), here.

2. SoundTracking

What Instagram is to photos, SoundTracking is trying to be for music. SoundTracking is a year-old app that allows you to share “music moments” with your friends. Sometimes this is simply pushing a notice of what you happen to be listening to at the moment, together with an image and a comment. At other times, users will send out music recommendations fit to something happening in the news–a Whitney Houston song on the occasion of her death, for instance, or something by The Clash during the London riots. The app integrates with iTunes, Rdio, and Spotify.

3. Meeteor

Several startups work from the insight that Facebook’s social graph can be leveraged for professional networking–perhaps even to a greater extent than LinkedIn. Meeteor, which helps you identify friends of friends who can help you get the job you want, is one such service. If Facebook wanted to take a swipe at LinkedIn for supremacy in the professional social network space, acquiring Meeteor might not be a bad idea.

4. Pearescope

Like anyone who follows social media, I’m constantly learning of apps that want to leverage location to help you meet new people. (MeetMoi NOW, for instance, I’ve called “a radar system for potential mates”; more recently, South by Southwest favorite Highlight has strayed into these waters.) When I first heard the pitch for Pearescope, though–at a New York Tech meetup, fittingly–I found myself thinking, “Finally, a location-based social app that isn’t creepy.” The fact is, meeting perfect strangers because they happen to be near you is weird. Meeting imperfect strangers, however–ones who share a friend on Facebook? That’s kind of normal-ish!

5. Trippy

“Social travel” startups are too numerous to list. But I’ll try: Tripl, Twigmore, Shiroube, Triptrotting… Why’s the smart money on Trippy? I can think of a few reasons. First, it doesn’t overreach: Trippy’s modest goal is to help you solicit travel advice from your social graph, something most of us already do on Facebook anyway. Second, its founder, J.R. Johnson, already has had success in the online travel space, with VirtualTourist. Third, Facebook money is already in Trippy: Randi Zuckerberg, Mark’s sister, is an investor.

6. The Wild Card

No, there isn’t a social poker startup called Wild Card (though if you use that name, contact me about licensing). My last startup is just a confession that the guessing game on which social sites flourish and which don’t is just that–a guessing game. Here are a few new and under-the-radar social networks of which I’m tempted to say, “That’s kind of boring.” In other worlds: they might be the next billion-dollar idea.