Short version: getting someone else to tell your story for you; incentivized word of mouth. Here's a few of the companies in the space directly and indirectly.
As someone in the influencer marketing space (100 players when I started, over 400 now), I view crowded spaces a bit differently. It implies, to me, a validation of consumer demand -- thus, those that can properly segment their offering to satisfy specific demands and execute more efficiently can pick up the available dollars.
intellifluence.com -- mine 6ixagency.com acorninfluence.com adly.com adretweet.com amazingdealsgroup.com amazingdealsnreviewsclub.com amazingreviewsclub.com amazonreviewernetwork.com ambassadher.com amzrc.com amzreviewtrader.com apexdrop.com audigent.com bazaarvoice.com beafreelanceblogger.com beatly.com behance.net bestsocialpr.com bigfra.me blanket.media blogacious.com blogdash.com bloggerbridge.com bloglovin.com blogmint.com blogprwire.com boostinsider.com brandbacker.com brandnew.io brandsnob.co buzzandgo.com buzzerati.com buzzmg.com buzzoole.com buzzstream.com bzzagent.com cartcam.com channelfactory.com channelpages.com cision.com/us/communication-cloud/ clearvoice.com/marketplace/ clevergirlscollective.com contentblvd.com contently.com contentmarketer.io contentmarketingstack.co contidio.com cooperatize.com copromote.com crowdcontent.com crowdly.com crowdtap.com cycle.media dealgogogo.com DealsForReviews.com delmondo.co distroscale.com dollardealreviews.com dollarreviewclub.com dotwriter.com dynamicsignal.com eleve.co elitedealclub.com evolvesinc.com exomatch.com exonia.net experticity.com exposely.com extrahyperactivenetwork.com famebit.com fanbytes.co.uk fango.me fanwise.com fashionprgirl.com fbamarkets.com fieldnation.com findyourinfluence.com flippersmack.com fluvip.com fromote.com fullbottle.co fullscreenmedia.co/creators galoremag.com getader.com getambassador.com getevangelized.com getlittlebird.com getreviewed.org getshout.io getswayd.com getwordperks.com giveawayservice.com gnackapp.com go.kred (empire.kred is the trading platform) go.nuvi.com grapevinelogic.com grinapps.com grouphigh.com gushcloud.com hashtagpaid.com helium.com hellosociety.com hirefluence.com hivency.com honestfew.com Hypetap.com hyprbrands.com ifluenz.com imagency.com Insense.pro influence.co influenceandco.com influencepeople.com influencer50.com influencerdb.net influencerconnect.com influencers.club influencersatwork.com influencersg.com influencive.com influenster.com influential.co influgram.com influicity.com inhouse.team innetwork.net insightpool.com instabrand.com iplus.co itrudeals.com izea.com jamballa.com juliusworks.com klear.com klout.com kontentcart.com kudobuzz.com lightningpages.com linkfromblog.com linqia.com littlesway.com loot-app.com markerly.com market.nmrkt.com marketplace.matadoru.com mavensocial.com mavrck.co media-part.com mediakix.com misfits.social musefind.com naritiv.com neoreach.com newlinenoosh.com newscred.com ngage.social Niche.co Nichify.com octoly.com onalytica.com onmogul.com outreachify.com paidpertweet.com patreon.com paypercontent.net peg.co phlanx.com pinchme.com pixability.com pond5.com popularpays.com powerreviews.com Preen.Me pro.iconosquare.com promoto.co productpilots.com producttestinggroup.com publicfast.com rainmakercollective.com reach-labs.com reachbird.io readypulse.com reelio.com revfluence.com reviewclub.io reviewkick.com reviews4success.com rewardstyle.com rightrelevance.com sampler.io scripted.com scrunch.com seedingup.com select.co shespeaksinc.com sideqik.com smallgirlspr.com smartviral.com snagshout.com Snapfluence.com Social-Friendzy.com sociabuzz.com socialix.com socialoholic.com smiley.socialmedialink.com socialrank.com socialyte.co sparkinfluence.net speakr.com splashscore.com sponsokit.com sponsoredmentions.com sponsoredsocial.com sponsoredtweets.com sprinklr.com stargazer.co studio71.com stylecoalition.com swarmcontent.com swayyem.com swaygroup.com swayops.com tailify.com takumi.com tapinfluence.com terakeet.com terareach.com theamplify.com thecirqle.com theexposure.co themidgame.com themobilemedialab.com thepivotpeople.com theshelf.com thinkrenegade.com thuzio.com tid.al tlistinc.com tomoson.com toprankmarketing.com topreviewzone.com totems.co tourn.co traackr.com trendrz.com Tribegroup.co triberr.com triond.com tru-signal.com tubularlabs.com tvinkle.com tweetangels.com uberzonclub.com unitedbloggers.no unityinfluence.com upfluence.com uptweet.com vaynermedia.com vazoola.com velocenetwork.com vidfall.com vidrocket.com vipon.com viralking.co viralnation.com vity.co vizified.com voltu.com webfluential.com webrand.com weconnectsocial.com weinfluence.co whalar.com whatsblog.com wibber.io withfluence.com workew.com writeraccess.com xberts.com yotpo.com youtube.com/yt/creators/ zerys.com zoomph.com zuberance.com