3 days back my website is at 48th page for the keyword search "mobile app development" in google, today morning i saw my page rank it is at 27th ,but after 8 hours in the evening if i look for my website in google search for keyword mentioned above is at again 48th rank how to improve this ,is am i done anything wrong i just posted my website links to some social networks from morning to evening that't the work i done only ,when will my site again comes to lowere page rank to 27 or in 20's
The term "mobile app development" is incredibly competitive. For instance AdWords lists it as High competitive and there is a suggested bid price of $52.61, indicating it is a highly valuable word that lots of people want to rank for. Given the highly competitive nature of that term, it isn't surprising you are seeing massive fluctuations in ranking - especially since you aren't ranking on page 1.
That said, there are some things to check for. Have you been building links for this term and could any of those link practices spammy or questionable? What changes have you made to your website recently--code, design, or content? See if you can find anything you've done recently that might be correlated to when these changes happen.
As well, what other terms does your website rank for? How are those terms performing? I'd check those to see if this term is just an outlier or if everything you rank for had similar fluctuations. That would tell you if this is specific to this phrase or if it is a problem with your site.