People only reveal the best sides of themselves online and thus creates a false image of their true personality.
References :

To be honest I can’t really say that online dating is worse than real life dating.

It would have been easier to write about how it is good for a relationship.

The worse thing is people being dishonest and posting fake photos, pretending to be someone else etc.
References :

I don’t see how online dating is bad for relationships. Meeting someone online rather than in person would not make that relationship different. People make themselves sound better when you meet in person to, you have to get to know anyone that you are dating. I’ve seen long distance relationships where people met online and had an online relationship for quite a while and then eventually met in person work out very well.
References :
http;//www.itsrainingmates.com
http://www.datingkeys.com