Mashable and TechCrunch are easily the biggest players out there when it comes to social media and tech blogs. Mashable first started out covering only social media and then expanded into other areas. The same happened to TechCrunch, it first started on technology and tech startups and then expanded into other areas.
Mashable seems to have more of a fun and playful theme, posting more generic content which many will enjoy, by doing this it's been able to build a massive following.
TechCrunch is quite the opposite. It mainly focuses on tech and business. It's target audience are tech specialists and entrepreneurs. By focusing on this niche it's been able to produce a massive following which is a great competitor to Mashable.
Unfortunately ever since TechCrunch was bought by AOL it has gone down hill a bit. With the original founder leaving with a bang. Content is still great on the site but the more personal touch to it all is disappearing slowly.
Reports of Mashable being bought are flying around. However, with Mashable not having much of a personal touch anyway, if it were to be bought I don't think there would be much of an impact.